Pandemic Live Attenuated Influenza Vaccine Research Articles

Early protection against influenza by pandemic live attenuated influenza vaccines

Live attenuated cold-adapted (ca) influenza vaccine (LAIV) is an effective tool for the control of influenza, most likely due to their ability to induce both humoral and cellular immune responses, easy application and relatively low manufacturing costs. Attenuated cold-adapted vaccine strains that have achieved a satisfactory balance between restricted replication and high immunogenicity are desirable. The immunogenicity of live attenuated vaccines may depend upon the interplay between its ability to induce pro-inflammatory cytokine responses and the relative sensitivity of the attenuated vaccine strain to an antiviral effect of these cytokines. To better understand the relationship between attenuation and induction of innate immunity as well as contribution of the early cytokine response to the relative immunogenicity of LAIVs, we have studied early protection induced by LAIV in vivo as well as early cytokine response in human cells macrophage origin in response to infection with vaccine strains or epidemic virus.
 The aim of this study was to investigate the early immune response and protective activity in female CBA mice intranasally immunized with cold-adapted influenza vaccine strains of different genome compositions of 5:3 or 6:2. For experimental infection pandemic influenza viruses A/South Africa/3626/13 (H1N1)pdm09 and A/New York/61/15 (H1N1)pdm09 were used to be administered to animals at a dose of 106 EID50 at day 3 after immunization (challenge infection). Although challenge viruses replicate at mice lungs at various, extend, on day 10 after immunization mice were protected from death from 60 up to 80%. Reassortants LAIV did not differ statistically on these levels.
 Study of the expression of IFN- and IFN- genes in human lung macrophage line cells THP-1 in vitro have shown that macrophages stimulated with vaccine strains with the genome formula 6:2 and 5:3, had a sufficient level of expression of these genes, comparable to that, as in infection with wild virus type A/South Africa/3626/13 (H1N1)pdm09. These data may indicate that surface proteins of influenza A virus are involved in the process of stimulation of the IFN- and IFN- genes.

Hemagglutinin head-specific responses dominate over stem-specific responses following prime boost with mismatched vaccines.

Broadly neutralizing Abs targeting the HA stem can provide broad protection against different influenza subtypes, raising the question of how best to elicit such Abs. We have previously demonstrated that vaccination with pandemic live-attenuated influenza vaccine (pLAIV) establishes immune memory for HA head-specific Abs. Here, we determine the extent to which matched versus mismatched LAIV-inactivated subunit vaccine (IIV) prime-boost vaccination elicits stem-specific memory B cells and Abs. We vaccinated African green monkeys with H5N1 pLAIV-pIIV or H5N1 pLAIV followed by seasonal IIV (sIIV) or with H5N1 pLAIV alone and measured Abs and HA-specific B cell responses. While we observed an increase in stem-specific memory B cells, head-specific memory B cell responses were substantially higher than stem-specific responses and were dominant even following boost with mismatched IIV. Neutralizing Abs against heterologous influenza viruses were undetectable. Head-specific B cells from draining lymph nodes exhibited germinal center markers, while stem-specific B cells found in the spleen and peripheral blood did not. Thus, although mismatched prime-boost generated a pool of stem-specific memory B cells, head-specific B cells and serum Abs substantially dominated the immune response. These findings have implications for including full-length native HA in prime-boost strategies intended to induce stem-specific Abs for universal influenza vaccination.

Intranasal Live Influenza Vaccine Priming Elicits Localized B Cell Responses in Mediastinal Lymph Nodes.

Pandemic live attenuated influenza vaccines (pLAIV) prime subjects for a robust neutralizing antibody response upon subsequent administration of a pandemic inactivated subunit vaccine (pISV). However, a difference was not detected in H5-specific memory B cells in the peripheral blood between pLAIV-primed and unprimed subjects prior to pISV boost. To investigate the mechanism underlying pLAIV priming, we vaccinated groups of 12 African green monkeys (AGMs) with H5N1 pISV or pLAIV alone or H5N1 pLAIV followed by pISV and examined immunity systemically and in local draining lymph nodes (LN). The AGM model recapitulated the serologic observations from clinical studies. Interestingly, H5N1 pLAIV induced robust germinal center B cell responses in the mediastinal LN (MLN). Subsequent boosting with H5N1 pISV drove increases in H5-specific B cells in the axillary LN, spleen, and circulation in H5N1 pLAIV-primed animals. Thus, H5N1 pLAIV primes localized B cell responses in the MLN that are recalled systemically following pISV boost. These data provide mechanistic insights for the generation of robust humoral responses via prime-boost vaccination.IMPORTANCE We have previously shown that pandemic live attenuated influenza vaccines (pLAIV) prime for a rapid and robust antibody response on subsequent administration of inactivated subunit vaccine (pISV). This is observed even in individuals who had undetectable antibody (Ab) responses following the initial vaccination. To define the mechanistic basis of pLAIV priming, we turned to a nonhuman primate model and performed a detailed analysis of B cell responses in systemic and local lymphoid tissues following prime-boost vaccination with pLAIV and pISV. We show that the nonhuman primate model recapitulates the serologic observations from clinical studies. Further, we found that pLAIVs induced robust germinal center B cell responses in the mediastinal lymph node. Subsequent boosting with pISV in pLAIV-primed animals resulted in detection of B cells in the axillary lymph nodes, spleen, and peripheral blood. We demonstrate that intranasally administered pLAIV elicits a highly localized germinal center B cell response in the mediastinal lymph node that is rapidly recalled following pISV boost into germinal center reactions at numerous distant immune sites.

H5N2 vaccine viruses on Russian and US live attenuated influenza virus backbones demonstrate similar infectivity, immunogenicity and protection in ferrets

The continued detection of zoonotic influenza infections, most notably due to the avian influenza A H5N1 and H7N9 subtypes, underscores the need for pandemic preparedness. Decades of experience with live attenuated influenza vaccines (LAIVs) for the control of seasonal influenza support the safety and effectiveness of this vaccine platform. All LAIV candidates are derived from one of two licensed master donor viruses (MDVs), cold-adapted (ca) A/Ann Arbor/6/60 or ca A/Leningrad/134/17/57. A number of LAIV candidates targeting avian H5 influenza viruses derived with each MDV have been evaluated in humans, but have differed in their infectivity and immunogenicity. To understand these differences, we generated four H5N2 candidate pandemic LAIVs (pLAIVs) derived from either MDV and compared their biological characteristics in vitro and in vivo. We demonstrate that all candidate pLAIVs, regardless of gene constellation and derivation, were comparable with respect to infectivity, immunogenicity, and protection from challenge in the ferret model of influenza. These observations suggest that differences in clinical performance of H5 pLAIVs may be due to factors other than inherent biological properties of the two MDVs.

Development of Clade-Specific and Broadly Reactive Live Attenuated Influenza Virus Vaccines against Rapidly Evolving H5 Subtype Viruses.

We have developed pandemic live attenuated influenza vaccines (pLAIVs) against clade 1 H5N1 viruses on an Ann Arbor cold-adapted (ca) backbone that induced long-term immune memory. In 2015, many human infections caused by a new clade (clade 2.2.1.1) of goose/Guangdong (gs/GD) lineage H5N1 viruses were reported in Egypt, which prompted updating of the H5N1 pLAIV. We explored two strategies to generate suitable pLAIVs. The first approach was to modify the hemagglutinin gene of a highly pathogenic wild-type (wt) clade 2.2.1.1 virus, A/Egypt/N03434/2009 (Egy/09) (H5N1), with its unmodified neuraminidase (NA) gene; this virus was designated Egy/09 ca The second approach was to select a low-pathogenicity avian influenza H5 virus that elicited antibodies that cross-reacted with a broad range of H5 viruses, including the Egypt H5N1 viruses, and contained a novel NA subtype for humans. We selected the low-pathogenicity A/duck/Hokkaido/69/2000 (H5N3) (dk/Hok/00) virus for this purpose. Both candidate vaccines were attenuated and immunogenic in ferrets, inducing antibodies that neutralized homologous and heterologous H5 viruses with different degrees of cross-reactivity; Egy/09 ca vaccine antisera were more specific for the gs/GD lineage viruses but did not neutralize recent North American isolates (clade 2.3.4.4), whereas antisera from dk/Hok/69 ca-vaccinated ferrets cross-reacted with clade 2.3.4.4 and 2.2.1 viruses but not clade 1 or 2.1 viruses. When vaccinated ferrets were challenged with homologous and heterologous H5 viruses, challenge virus replication was reduced in the respiratory tract. Thus, the two H5 pLAIV candidates are suitable for clinical development to protect humans from infection with different clades of H5 viruses.IMPORTANCE In response to the continuing evolution of H5N1 avian influenza viruses and human infections, new candidate H5 live attenuated vaccines were developed by using two different approaches: one targeted a specific circulating strain in Egypt, and the other was based on a virus that elicits broadly cross-reactive antibodies against a wide range of H5 viruses. Both candidate vaccines were immunogenic and exhibited protective efficacy in ferrets. Our study permits a comparison of the two approaches, and the data support the further development of both vaccine viruses to optimally prepare for the further spread of clade 2.2.1 or 2.3.4.4 viruses.

Evaluation of the Safety and Immunogenicity of a Candidate Pandemic Live Attenuated Influenza Vaccine (pLAIV) Against Influenza A(H7N9).

Background. We evaluated a candidate A/Anhui/2013(H7N9) pandemic live attenuated influenza vaccine (pLAIV) in healthy adults, and assessed the ability of 1 or 2 doses to induce immune memory. Methods. Healthy subjects in 2 age groups (18–49 years and 50–70 years) with undetectable hemagglutination-inhibiting (HAI) antibody to H7N9 were enrolled. Younger subjects received either 1 or 2 intranasal doses of 107.0 fluorescent focus units of A/Anhui/1/2013 pLAIV, while older subjects received a single dose. All subjects received a single 30-µg dose of unadjuvanted, antigenically matched A/Shanghai2/2013(H7N9) pandemic inactivated influenza vaccine (pIIV) 12 weeks after their first dose of pLAIV. Results. Both vaccines were well tolerated. Serum HAI antibody responses were detected in 0 of 32 younger subjects and 1 of 17 older subjects after 1 dose of pLAIV and in 2 of 16 younger subjects after a second dose. Strong serum antibody responses were detected after a single subsequent dose of pIIV that was broadly reactive against H7 influenza viruses. Conclusions. An A(H7N9) pLAIV candidate was safe in both age groups. Priming with pLAIV resulted in responses to subsequent pIIV that exceeded those seen in naive subjects in previous reports. The A(H7N9) pLAIV induces strong immune memory that can be demonstrated by exposure to subsequent antigenic challenge. Clinical Trials Registration. NCT01995695 and NCT02274545.

H2N2 live attenuated influenza vaccine is safe and immunogenic for healthy adult volunteers

H2N2 influenza viruses have not circulated in the human population since 1968, but they are still being regularly detected in the animal reservoir, suggesting their high pandemic potential. To prepare for a possible H2N2 pandemic, a number of H2N2 vaccine candidates have been generated and tested in preclinical and clinical studies. Here we describe the results of a randomized, double-blind placebo-controlled phase 1 clinical trial of an H2N2 live attenuated influenza vaccine (LAIV) candidate prepared from a human influenza virus isolated in 1966. The vaccine candidate was safe and well-tolerated by healthy adults, and did not cause serious adverse events or an increased rate of moderate or severe reactogenicities. The H2N2 vaccine virus was infectious for Humans. It was shed by 78.6% and 74.1% volunteers after the first and second dose, respectively, most probably due to the human origin of the virus. Importantly, no vaccine virus transmission to unvaccinated subjects was detected during the study. We employed multiple immunological tests to ensure the adequate assessment of the H2N2 pandemic LAIV candidate and demonstrated that the majority (92.6%) of the vaccinated subjects responded to the H2N2 LAIV in one or more immunological tests, including 85.2% of subjects with antibody responses and 55.6% volunteers with cell-mediated immune responses. In addition, we observed strong correlation between the H2N2 LAIV virus replication in the upper respiratory tract and the development of antibody responses.

New Methodological Approaches in The Development of Russian Live Attenuated Vaccine for Pandemic Influenza

Avian influenza viruses remain a major pandemic threat. In response to this threat, a number of pandemic vaccines have been developed. The objective of this paper is to review and summarize data from preclinical and clinical evaluation of Russian live attenuated influenza vaccines (LAIVs) against pandemic influenza based on cold-adapted A/Leningrad/134/17/57 (H2N2) master donor virus (MDV). The described LAIVs consist of reassortant viruses of 6:2 and 7:1 genomic composition (6 MDV genes: 2 WT genes and 7 MDV genes: 1 WT gene, respectively). Despite the differences in their genomic composition (6:2 or 7:1), LAIV candidates of H5, H7 and H2 subtypes acquired temperature sensitivity, cold-adaptation, and attenuation for different animal models. In addition, they were safe and immunogenic for healthy adult volunteers. The collected data indicate that 7:1 reassortants carrying HA genes of potentially pandemic viruses and the remaining genes from the MDV might be preferable pandemic LAIV candidates.

Live attenuated H7N7 influenza vaccine primes for a vigorous antibody response to inactivated H7N7 influenza vaccine

BackgroundH7 influenza viruses have emerged as potential pandemic threat. We evaluated the safety and immunogenicity of two candidate H7 pandemic live attenuated influenza vaccines (pLAIV) and their ability to prime for responses to an unadjuvanted H7 pandemic inactivated influenza vaccine (pIIV). MethodsHealthy seronegative adults received two doses of A/Netherlands/219/03 (H7N7) or one dose of A/chicken/British Columbia/CN-6/04 (H7N3) pLAIV all given as 107.5 50% tissue culture infective doses (TCID50) intranasally. A subset of subjects received one 45μg dose of H7N7 pIIV containing the A/Mallard/Netherlands/12/2000 HA intramuscularly 18–24 months after pLAIV. Viral shedding was assessed by culture and real-time polymerase chain reaction (rRT-PCR), B cell responses following pLAIV were evaluated by ELISPOT and flow cytometry. Serum antibody was assessed by hemagglutination-inhibition (HAI), microneutralization (MN) and ELISA assays after each vaccine. ResultsSerum HAI or MN responses were not detected in any subject following one or two doses of either H7 pLAIV, although some subjects had detectable H7 specific B cells after vaccination. However, 10/13 subjects primed with two doses of H7N7 pLAIV responded to a subsequent dose of the homologous H7N7 pIIV with high titer HAI and MN antibody that cross-reacted with both North American and Eurasian lineage H7 viruses, including H7N9. In contrast, naïve subjects and recipients of a single dose of the mismatched H7N3 pLAIV did not develop HAI or MN antibody after pIIV. ConclusionsWhile pLAIVs did not elicit detectable serum MN or HAI antibody, strain-specific pLAIV priming established long term immune memory that was cross-reactive with other H7 influenza strains. Understanding the mechanisms underlying priming by pLAIV may aid in pandemic vaccine development.

The Matrix Gene Segment Destabilizes the Acid and Thermal Stability of the Hemagglutinin of Pandemic Live Attenuated Influenza Virus Vaccines

The threat of future influenza pandemics and their potential for rapid spread, morbidity, and mortality has led to the development of pandemic vaccines. We generated seven reassortant pandemic live attenuated influenza vaccines (pLAIVs) with the hemagglutinin (HA) and neuraminidase (NA) genes derived from animal influenza viruses on the backbone of the six internal protein gene segments of the temperature sensitive, cold-adapted (ca) A/Ann Arbor/60 (H2N2) virus (AA/60 ca) of the licensed seasonal LAIV. The pLAIV viruses were moderately to highly restricted in replication in seronegative adults; we sought to determine the biological basis for this restriction. Avian influenza viruses generally replicate at higher temperatures than human influenza viruses and, although they shared the same backbone, the pLAIV viruses had a lower shutoff temperature than seasonal LAIV viruses, suggesting that the HA and NA influence the degree of temperature sensitivity. The pH of HA activation of highly pathogenic avian influenza viruses was greater than human and low-pathogenicity avian influenza viruses, as reported by others. However, pLAIV viruses had a consistently higher pH of HA activation and reduced HA thermostability compared to the corresponding wild-type parental viruses. From studies with single-gene reassortant viruses bearing one gene segment from the AA/60 ca virus in recombinant H5N1 or pH1N1 viruses, we found that the lower HA thermal stability and increased pH of HA activation were associated with the AA/60 M gene. Together, the impaired HA acid and thermal stability and temperature sensitivity likely contributed to the restricted replication of the pLAIV viruses we observed in seronegative adults. There is increasing evidence that the HA stability of influenza viruses depends on the virus strain and host species and that HA stability can influence replication, virulence, and transmission of influenza A viruses in different species. We investigated the HA stability of pandemic live attenuated influenza vaccine (pLAIV) viruses and observed that the pLAIV viruses consistently had a less stable HA than the corresponding wild-type influenza viruses. The reduced HA stability and temperature sensitivity of the pLAIV viruses may account for their restricted replication in clinical trials.

Live attenuated influenza vaccine.

Cold-adapted Ann Arbor based live attenuated influenza vaccine (LAIV) has been available in the USA since 2003. The vaccine is efficacious against influenza infection. Features of LAIV include: easy administration suitable for mass immunization, cross-reactivity to drifted strains for broader coverage, and establishment of herd immunity for control of influenza spread. Annual seasonal LAIV now contains four strains against influenza A H1N1, H3N2, influenza B-Victoria, and B-Yamagata lineages that are co-circulating in humans. LAIV played a significant role in protecting the public from the 2009 H1N1 pandemic and has been evaluated for pandemic preparedness. Pandemic vaccines including influenza H2, H5, H6, H7, and H9 subtypes have been produced and evaluated in preclinical and small-scale phase I clinical studies. This review summarizes the current status and perspectives of seasonal and pandemic LAIV.

Genome composition analysis of reassortant influenza viruses used in seasonal and pandemic live attenuated influenza vaccine

The cold-adapted, temperature sensitive and attenuated influenza master donor viruses A/Leningrad/134/17/57 (H2N2) and B/USSR/ 60/69 were used to generate the vaccine viruses to be included in live attenuated influenza vaccine. These vaccine viruses typically are 6:2 reassortant viruses containing the surface antigens hemagglutinin and neuraminidase of current wild type influenza A and influenza B viruses with the gene segments encoding the internal viral proteins, and conferring the cold-adapted, temperature sensitive and attenuated phenotype, being inherited from the master donor viruses. The 6:2 reassortant viruses were selected from co-infections between master donor virus and wild type viruses that theoretically may yield as many as 256 combinations of gene segments and thus 256 genetically different viruses. As the time to generate and isolate vaccine viruses is limited and because only 6:2 reassortant viruses are allowed as vaccine viruses, screening needs to be both rapid and unambiguous. The screening of the reassortant viruses by RT-PCRs using master donor virus and wild type virus specific primer sets was described to select both influenza A and influenza B 6:2 reassortant viruses to be used in seasonal and pandemic live attenuated vaccine.

Live attenuated pandemic influenza vaccine: Clinical studies on A/17/California/2009/38 (H1N1) and licensing of the Russian-developed technology to WHO for pandemic influenza preparedness in developing countries

In February 2009, Nobilon granted the World Health Organization (WHO) a non-exclusive licence to develop, register, manufacture, use and sell seasonal a pandemic live attenuated influenza vaccine (LAIV) produced on embryonated chicken eggs. WHO was permitted to grant sub-licences to vaccine manufacturers in developing countries within the framework of its influenza vaccine technology transfer initiative. In parallel, the Institute of Experimental Medicine (IEM), Russia, concluded an agreement with WHO for the supply of Russian LAIV reassortants for use by these manufacturers.Also in 2009, IEM carried out a study on a novel A/17/California/2009/38 (H1N1) pandemic LAIV candidate derived from the pandemic-related A/California/07/2009 (H1N1) influenza virus and the attenuated A/Leningrad/134/17/57 (H2N2) master donor virus, using routine reassortant technique in embryonated chicken eggs. Following successful preclinical studies in eggs and in ferrets, a double-blind, controlled, randomized clinical trial was carried out in immunologically naïve study participants between 12–18 and 18–60 years old. Collectively, the immunogenicity data (haemagglutinin inhibition test, ELISA and cytokine tests for the detection of memory T cells) support the use of a single dose of the pandemic H1N1 LAIV in 12–60 year olds. The outcome of the studies showed no significant adverse reactions attributable to the vaccine, and suggests that the vaccine is as safe and immunogenic as seasonal influenza vaccines. Importantly, it was clearly demonstrated that reliance on the HAI assay alone is not recommended for testing LAIV.To date, via the licence agreement with WHO, the H1N1 LAIV has been transferred to the Government Pharmaceutical Organization in Thailand, the Serum Institute of India, and the Zhejiang Tianyuan Bio-Pharmaceutical Co., Ltd. in China.

Development of influenza vaccine production capacity by the Government Pharmaceutical Organization of Thailand: Addressing the threat of an influenza pandemic

In 2005, a year after highly pathogenic avian influenza outbreaks in Thailand, the Thai Government issued a National Strategy Plan for Pandemic Influenza Preparedness, a major objective of which was the domestic production of seasonal influenza vaccine. It was considered that sustained influenza vaccine production was the best guarantee of a pandemic vaccine in the event of a future pandemic. The Government decided to provide funds to establish an industrial-scale influenza vaccine production plant, and gave responsibility for this challenging project to the Government Pharmaceutical Organization (GPO). In 2007, with support from the World Health Organization (WHO), the GPO started to develop egg-based, trivalent inactivated influenza vaccine (IIV) in a renovated pilot plant. In early 2009, during the second year of the project, the GPO turned its attention to develop a pandemic live attenuated influenza vaccine (PLAIV) against the influenza A (H1N1) virus. By December 2010, the H1N1 PLAIV had successfully completed Phase II clinical trials and was awaiting registration approval from the Thai Food and Drug Administration (TFDA). The GPO has also started to develop an H5N2 PLAIV, which is expected to enter clinical trials in January 2011. The next step in 2011 will be the development and clinical evaluation of seasonal LAIV. To meet the needs of the national seasonal influenza vaccination programme, the GPO aims to produce 2 million doses of trivalent IIV in 2012 and progressively increase production to the maximum annual capacity of 10 million doses. This article relates how influenza vaccine production capacity was developed and how major challenges are being met in an expeditious manner, with strong local and global commitment.

Seasonal influenza infection and live vaccine prime for a response to the 2009 pandemic H1N1 vaccine

The robust immune response to a single dose of pandemic 2009 H1N1 vaccine suggests that a large segment of the population has been previously primed. We evaluated the effect of seasonal (s) H1N1 infection, s-trivalent inactivated vaccine (s-TIV), and trivalent s-live attenuated influenza vaccine (s-LAIV) before immunization with a pandemic live attenuated influenza vaccine (p-LAIV) in mice. We compared serum and mucosal antibody and pulmonary CD8 and CD4 responses and the virologic response to challenge with a wild-type 2009 pandemic H1N1 (p-H1N1) virus. Two doses of p-LAIV induced cellular immune and robust ELISA and neutralizing antibody responses that were associated with complete protection from p-H1N1 challenge. A single dose of p-LAIV induced a cellular response and ELISA but not a neutralizing antibody response, and incomplete protection from p-H1N1 virus challenge. Primary infection with s-H1N1 influenza virus followed by a dose of p-LAIV resulted in cross-reactive ELISA antibodies and a robust cellular immune response that was also associated with complete protection from p-H1N1 virus challenge. A lower-magnitude but similar response associated with partial protection was seen in mice that received a dose of s-LAIV followed by p-LAIV. Mice that received a dose of s-TIV followed by p-LAIV did not show any evidence of priming. In summary, prior infection with a seasonal influenza virus or s-LAIV primed mice for a robust response to a single dose of p-LAIV that was associated with protection equivalent to two doses of the matched pandemic vaccine.

Concomitant administration of seasonal trivalent and pandemic monovalent H1N1 live attenuated influenza vaccines

To the editor: In the United States, live attenuated influenza vaccine (LAIV) will play a prominent role in the novel A(H1N1) pandemic vaccination campaign; more than 40 million doses have been purchased by the US Government. Additionally, approximately 10 million doses of seasonal trivalent LAIV will be available for use during the 2009–2010 influenza season. Seasonal trivalent LAIV is currently approved for use in eligible individuals aged 2–49 years in the United States, South Korea and Hong Kong. Because some US populations have been recommended to receive both seasonal trivalent and pandemic monovalent vaccine during the 2009–2010 influenza season, questions have arisen regarding concomitant vaccination with seasonal trivalent and pandemic monovalent LAIV. Historically, recommendations of the Centers for Disease Control and Prevention Advisory Committee on Immunization Practices (ACIP) have stated, ‘In the absence of specific data indicating interference, following ACIP’s general recommendations for vaccination is prudent. Inactivated vaccines do not interfere with the immune response to other inactivated vaccines or to live vaccines. Inactivated or live vaccines can be administered simultaneously with LAIV. However, after administration of a live vaccine, at least 4 weeks should pass before another live vaccine is administered’. 1 Specific guidance was published in 2009 related to vaccination with pandemic monovalent vaccines, which stated, ‘Simultaneous administration of inactivated vaccines against seasonal and novel influenza A (H1N1) viruses is permissible if different anatomic sites are used. However, simultaneous administration of LAIV against seasonal and novel influenza A (H1N1) virus is not recommended’. 2 It was subsequently noted that the recommendation against simultaneous intranasal administration of seasonal and pandemic LAIV was because of theoretical concerns for potential interference between the vaccines. 3 Here, we report the results of a study designed to examine the potential for interference following concomitant administration of seasonal and pandemic LAIV in ferrets, a widely accepted and relevant animal model often used to examine influenza virus and influenza vaccine immunogenicity, including annual World Health Organization and US Food and Drug Administration evaluation of candidate vaccine strains. 4 , 5 , 6 , 7 , 8 Twenty 8‐week‐old male ferrets (Triple F Farms, Sayre, PA, USA) seronegative for all four influenza strains were used in the study. One cohort (n = 4) was inoculated intranasally with a 0·2‐ml dose (0·1 ml per nostril) of seasonal trivalent LAIV containing 106·5−7·5 fluorescent focus units (FFU) of each of the three cold‐adapted, temperature‐sensitive vaccine strains recommended for inclusion in the 2009–2010 vaccine: A/South Dakota/6/2007 (H1N1) (A/Brisbane/59/2007‐like), A/Uruguay/716/2007 (H3N2) (A/Brisbane/10/2007‐like) and B/Brisbane/60/2008. A second cohort (n = 4) was inoculated intranasally with 106·5−7·5 FFU per dose of the cold‐adapted, temperature‐sensitive 2009 H1N1 monovalent vaccine, A/California/07/2009 (H1N1). A third group (n = 12) was inoculated intranasally with pandemic monovalent LAIV followed by seasonal trivalent LAIV approximately 15 seconds later. This third cohort included more animals to allow for further division into three subgroups to investigate second‐dose responses if interference was observed. Sera were collected weekly, and the immunogenicity and kinetics of the immune response were determined by strain‐specific serum hemagglutination inhibition (HAI) on days 0 (pre‐dose), 14 and 28 post‐inoculation using standard methods with 0·5% chicken erythrocytes. Cold‐adapted virus antigen was used for A/California/07/2009; wild‐type antigen was used for seasonal strains. Serum antibody responses to the four vaccine strains are depicted in Figure 1. All strains were immunogenic and strain‐specific responses were statistically similar in the cohorts receiving seasonal vaccine, pandemic vaccine and both vaccines concomitantly. No interference with concomitant vaccination was observed at either 14 or 28 days post‐vaccination. LAIV viruses replicate primarily in the ciliated epithelial cells of the nasopharyngeal mucosa to induce immune responses via mucosal immunoglobulin A (IgA), serum IgG and cellular immunity. Serum antibody responses are not a correlate of protection; in fact, some studies have shown protection in the absence of significant antibody responses. 9 , 10 , 11 However, consistent with their use in the present study, HAI responses have been used to establish comparability of different LAIV formulations and evaluate concomitant vaccination regimens. 12 , 13 , 14 Figure 1 Mean (log2) hemagglutination inhibition (HAI) titer for each vaccine strain 14 (A) and 28 days (B) after one dose of seasonal trivalent or pandemic monovalent H1N1 influenza vaccine or one dose of each vaccine administered concurrently. ... These data in ferrets indicate the development of a robust and consistent immune response at 14 and 28 days post‐inoculation with seasonal trivalent and pandemic monovalent H1N1 vaccines. There was no evidence of interference in the cohort receiving concomitant seasonal and pandemic vaccination. These data may help inform vaccination recommendations for the 2009–2010 influenza season in the United States. It should be noted that no data regarding concomitant vaccination exist for humans at this time.