Vaccine Failure Rate Research Articles

Seroepidemiology of measles and rubella among Hong Kong young adults and the humoral responses of an MMR booster among participants with low antibody levels.

Some individuals may not retain adequate immunity against measles and rubella years after two doses of measles, mumps, and rubella (MMR) vaccination due to vaccine failure. This study aimed to investigate the rates of vaccine failure and seroconversion by administering an MMR booster to young adults. We first assessed measles and rubella antibody levels using the Luminex multiplex assay, VIDAS IgG assay, and plaque reduction neutralization test (PRNT) among individuals aged 18-30 years old who had received two doses of MMR vaccine. Participants with low measles and/or rubella antibody levels as confirmed by VIDAS received an MMR booster. Antibody levels were measured at 1-month post-booster. Among 791 participants, the measles and rubella seroprevalence rates were 94.7% (95% CI: 92.9%-96.0%) and 97.3% (95% CI: 96.0%-98.3%), respectively. Lower seroprevalence rates were observed among older participants. 113 participants who received an MMR booster acquired higher measles and rubella antibody levels at 1-month post-booster compared to baseline. Although measles and rubella vaccine failures were observed among 5.3% and 2.7% of young adults, respectively, an MMR booster triggered a significant antibody response.

Effect of COVID-19 Vaccines on the Prevention and Severity of Omicron Strain in Liver Transplant Patient

Background: Because it is still not possible to accurately determine whether the injected vaccines affect the disease incidence and mortality or not in the newly diagnosed strains, the present study aimed to investigate the effect of injected coronavirus disease 2019 (COVID-19) vaccines on the mortality rate among liver transplant patients infected with COVID-19 in Mashhad, Iran. Methods: This prospective cross-sectional study was conducted on liver transplant patients with moderate to severe COVID-19 referred to Montaseriyeh Hospital, Mashhad, Iran, from December 2021 to March 2022. The relationship between mortality due to Omicron strain was assessed with various variables. Results: In general, 97 liver transplant recipients were entered into the present study. Vaccine failure was reported in 43.5% of liver recipients. About 30% of the patients had not received any COVID-19 vaccination, and 2.9%, 40%, and 27.1% had received one, two, and three dosages of COVID-19 vaccination, respectively. Infection with COVID-19 was the cause of mortality in 11.3% of patients. No significant relationship was reported between mortality and the consumption of immunosuppressive agents (P > 0.05). Multiple linear regression showed that the number of received vaccine dosages was predictive of mortality due to infection with the Omicron variant in liver recipients (β = 0.13; P < 0.005). Conclusions: It was found that mortality due to COVID-19 vaccination was higher among the patients with fewer COVID-19 vaccination dosages and, consequently, could be related to vaccine-induced immunity in liver transplant recipients. However, due to the high vaccine failure rate, it seems that neutralizing antibody activity against Omicron variants is high.

Forward-backward and period doubling bifurcations in a discrete epidemic model with vaccination and limited medical resources.

A discrete epidemic model with vaccination and limited medical resources is proposed to understand its underlying dynamics. The model induces a nonsmooth two dimensional map that exhibits a surprising array of dynamical behavior including the phenomena of the forward-backward bifurcation and period doubling route to chaos with feasible parameters in an invariant region. We demonstrate, among other things, that the model generates the above described phenomena as the transmission rate or the basic reproduction number of the disease gradually increases provided that the immunization rate is low, the vaccine failure rate is high and the medical resources are limited. Finally, the numerical simulations are provided to illustrate our main results.

Incorporating economic constraints for optimal control of immunizing infections.

It is well-known that the interruption of transmission of a disease can be achieved, provided the vaccinated population reaches a threshold depending on, among others, the efficacy of vaccines. The purpose of this paper is to address the optimal vaccination strategy by imposing the economic constraints. In particular, an S--(I,V)--S model used to describe the spreading of the disease in a well-mixed population and a cost function consisting of vaccination and infection costs are proposed. The well-definedness of the above-described modeling is provided. We were then able to provide an optimal strategy to minimize the cost for all parameters. In particular, the optimal vaccination level to minimize the cost can be completely characterized for all parameters. For instance, the optimal vaccination level can be classified by the magnitude of the failure rate of the vaccine with other parameters being given. Under these circumstances, the optimal strategy to minimize the cost is roughly to eliminate the disease locally (respectively, choose an economic optimum resulting in not to wipe out the disease completely or take no vaccination for anyone) provided the vaccine failure rate is relatively small (respectively, intermediate or large). Numerical simulations to illustrate our main results are also provided. Moreover, the data collected at the height of the Covid-19 pandemic in Taiwan are also numerically simulated to provide the corresponding optimal vaccination strategy.

Targeting memory T cell metabolism to improve immunity.

Vaccination affords protection from disease by activating pathogen-specific immune cells and facilitating the development of persistent immunologic memory toward the vaccine-specific pathogen. Current vaccine regimens are often based on the efficiency of the acute immune response, and not necessarily on the generation of memory cells, in part because the mechanisms underlying the development of efficient immune memory remain incompletely understood. This Review describes recent advances in defining memory T cell metabolism and how metabolism of these cells might be altered in patients affected by mitochondrial diseases or metabolic syndrome, who show higher susceptibility to recurrent infections and higher rates of vaccine failure. It discusses how this new understanding could add to the way we think about immunologic memory, vaccine development, and cancer immunotherapy.

A kinetic model considering the decline of antibody level and simulation about vaccination effect of COVID-19.

We build a model that consider the falling antibody levels and vaccination to assess the impact of falling antibody levels and vaccination on the spread of the COVID-19 outbreak, and simulate the influence of vaccination rates and failure rates on the number of daily new cases in England. We get that the lower the vaccine failure rate, the fewer new cases. Over time, vaccines with low failure rates are more effective in reducing the number of cases than vaccines with high failure rates and the higher the vaccine efficiency and vaccination rate, the lower the epidemic peak. The peak arrival time is related to a boundary value. When the failure rate is less than this boundary value, the peak time will advance with the decrease of failure rate; when the failure rate is greater than this boundary value, the peak time is delayed with the decrease of failure rate. On the basis of improving the effectiveness of vaccines, increasing the vaccination rate has practical significance for controlling the spread of the epidemic.

421. Effect of SARs-Cov-2 mRNA Vaccination in Healthcare Workers with Household COVID Exposure

BackgroundHealthcare workers have experienced a significant burden of COVID-19 disease. COVID mRNA vaccines have shown great efficacy in prevention of severe disease and hospitalization due to COVID infection, but limited data is available about acquisition of infection and asymptomatic viral shedding.MethodsFully vaccinated healthcare workers at a tertiary-care academic medical center in Omaha Nebraska who reported a household exposure to COVID-19 infection are eligible for a screening program in which they are serially screened with PCR but allowed to work if negative on initial test and asymptomatic. Serial screening by NP swab was completed every 5-7 days, and workers became excluded from work if testing was positive or became symptomatic. ResultsOf the 94 employees who were fully vaccinated at the time of the household exposure to COVID-19 infection, 78 completed serial testing and were negative. Sixteen were positive on initial or subsequent screening. Vaccine failure rate of 17.0% (16/94). Healthcare workers exposed to household COVID positive contact ConclusionHigh risk household exposures to COVID-19 infection remains a significant potential source of infections in healthcare workers even after workers are fully vaccinated with COVID mRNA vaccines especially those with contact to positive domestic partners. Disclosures All Authors: No reported disclosures

Hepatitis B in the Northern Territory: insights into the changing epidemiology of an ancient condition.

BackgroundAboriginal and Torres Strait Islander people are disproportionately affected by hepatitis B virus (HBV) infection. A proposed mismatch between standard vaccines and the HBV/C4 sub‐genotype prevalent in Aboriginal people in the Northern Territory (NT) may reduce vaccine effectiveness.AimsTo determine HBV prevalence in the NT by Indigenous status and to explore patterns of immunity following implementation of universal vaccination, using a large longitudinal diagnostic dataset.MethodsA retrospective analysis of all available HBV serology results in the NT from 1991 to 2011 was conducted, with HBV prevalence and vaccination status analysed according to adigenous status, age and sex using individuals' patterns of HBsAg, anti‐HBs and anti‐HBc serology over repeated tests.Results100 790 individuals were tested (33.4% Indigenous) between 1991 and 2011 (26.1% of the 2011 NT population), with a total of 211 802 tests performed. In 2011, the proportion of HBV positive individuals in the NT was 3.17% (5.22% in Indigenous populations) compared to previous 2011 estimates of 1.70% (3.70% in Indigenous populations). The vaccine failure rate was lower than expected with only one presumed vaccinated person subsequently developing HBsAg positivity (0.02%). Evidence of suboptimal vaccine efficacy by breakthrough anti‐HBc positivity in vaccinated individuals was demonstrated in 3.1% of the vaccinated cohort, of which 86.4% identified as Indigenous (HR 1.17). No difference in HBeAg positivity or seroconversion was observed between Indigenous and non‐Indigenous individuals living with CHB.ConclusionsThe burden of CHB in Indigenous people in the NT has previously been underestimated. A higher HBV prevalence in the NT than described in previous cross‐sectional studies was found, including a higher prevalence in Indigenous people. Evidence of suboptimal vaccine efficacy was demonstrated predominantly in Indigenous individuals.

Q fever vaccine efficacy and occupational exposure risk in Queensland, Australia: A retrospective cohort study

Q-VAX® is a vaccine used to prevent Q fever. Administration of the vaccine is complicated by the need to ensure, using intradermal and serological tests, that individuals have no prior immunity. Previous studies suggest that the vaccine is highly efficacious and long-lasting in adults. However, there has been no systematic follow-up of vaccine efficacy and the longevity of immunity using population-level data. We aimed to investigate the vaccine failure rate and duration of immunity in previously vaccinated individuals. We formulated a retrospective cohort study design within a linked data. We used a Q fever vaccination registry linked to Q fever notifications and hospital admissions (1991–2016) in the state of Queensland, which has Australia’s highest incidence of Q fever. Q-VAX® failure was defined as occurrence of Q fever > 14 days’ after vaccination. The incidence of Q fever in vaccinated and unvaccinated individuals was 5.40 (95% CI: 3.65, 7.72) and 89.50 (95% CI: 70.50, 112.00]) per 100,000 person-years of follow-up, respectively. The hazard ratio (HR) for Q fever was 0.07 (95% CI: 0.04, 0.10) in non-immune vaccinated compared with immune unvaccinated individuals. The overall vaccine effectiveness was found to be 94.37% suggesting that Q-VAX® is highly effective at preventing Q fever. However, the greater incidence observed in unvaccinated individuals considered immune during the pre-vaccination screening may suggest that pre-vaccination screening is sub-optimal among this study population.

The Efficacy of the Quadrivalent Human Papillomavirus Vaccine in Girls and Women Living With Human Immunodeficiency Virus.

Human papillomavirus (HPV) vaccination is safe and efficacious in women without human immunodeficiency virus (HIV). Although good immunogenicity has been observed in women living with HIV (WLWH), efficacy data in this population are needed. We enrolled 420 females aged ≥9 years (range, 9-65) living with HIV. Participants were to receive 3 doses of qHPV vaccine (0/2/6 months). The main endpoint was vaccine failure (ie, incident persistent qHPV infection, cervical intraepithelial neoplasia of grade 2 or higher [CIN2+], or genital warts). We compared these rates to published rates in vaccinated and unvaccinated women without HIV as well as unvaccinated WLWH. Among 279 eligible women, median follow-up was 2 years. In the intention-to-treat population, the incidence rate (IR) of persistent qHPV (HPV6/11/16/18) was 2.3 per 100 person-years (/100PY) (95% confidence interval [CI], 1.1-4.1), and IR of genital warts was 2.3/100PY (95% CI, 1.2-4.1). In the per-protocol efficacy population, IR of persistent qHPV was 1.0/100PY (95% CI, 0.3-2.6) and of genital warts was 1.0/100PY (95% CI, 0.3-2.5). No cases of CIN2+ occurred. Reported rates of qHPV-related infection and disease within vaccinated women without HIV, unvaccinated women without HIV, and vaccinated WLWH: 0.1 (95% CI, 0.02-0.03), 1.5 (95% CI, 1.1-2.0), and 1.2 (95% CI, 0.2-3.4) /100PY, respectively. The rate of persistent qHPV among vaccinated WLWH was lower than among unvaccinated WLWH (2.3 vs 6.0/100PY). Vaccinated WLWH may be at higher risk for vaccine failure than vaccinated women without HIV. However, overall rates of vaccine failure were low, and rates of persistent qHPV were lower than in unvaccinated WLWH.

Correlation between measles vaccine doses: implications for the maintenance of elimination.

Measles eradication efforts have been successful at achieving elimination in many countries worldwide. Such countries actively work to maintain this elimination by continuing to improve coverage of two routine doses of measles vaccine following measles elimination. While improving measles vaccine coverage is always beneficial, we show, using a steady-state analysis of a dynamical model, that the correlation between populations receiving the first and second routine dose also has a significant impact on the population immunity achieved by a specified combination of first and second dose coverage. If the second dose is administered to people independently of whether they had the first dose, high second-dose coverage improves the proportion of the population receiving at least one dose, and will have a large effect on population immunity. If the second dose is administered only to people who have had the first dose, high second-dose coverage reduces the rate of primary vaccine failure, but does not reach people who missed the first dose; this will therefore have a relatively small effect on population immunity. When doses are administered dependently, and assuming the first dose has higher coverage, increasing the coverage of the first dose has a larger impact on population immunity than does increasing the coverage of the second. Correlation between vaccine doses has a significant impact on the level of population immunity maintained by current vaccination coverage, potentially outweighing the effects of age structure and, in some cases, recent improvements in vaccine coverage. It is therefore important to understand the correlation between vaccine doses as such correlation may have a large impact on the effectiveness of measles vaccination strategies.

Ιnfluenza A(H3N2) genetic variants in vaccinated patients in northern Greece

BackgroundInfluenza A(H3N2) viruses predominated during the influenza 2016/2017 season and showed extensive genetic diversification. A high vaccination failure rate was noticed during the 2016/17 season in Greece, especially among the elderly. ObjectivesThe scope of the study was to investigate the genetic characteristics of A(H3N2) circulating viruses and viruses detected in vaccinated patients. Study designVirus samples originated from vaccinated and unvaccinated patients, obtained at the National Influenza Centre for northern Greece. Phylogenetic analysis and comparison of the haemagglutinin gene of the viruses to the vaccine virus A/Hong Kong/4801/2014 was performed. ResultsThe majority of analysed viruses are clustering in the genetic clade 3C.2a, and in a newly emerged subclade, designated as 3C.2a1. The highest proportion of viruses detected in vaccinated patients fell into a distinct subcluster within the 3C.2a1 subclade, which is characterised by the amino acid substitutions N122D and T135K in haemagglutinin. ConclusionsViruses that belong to the 3C.2a clade are generally considered to resemble antigenically to the northern hemisphere vaccine component A/Hong Kong/4801/2014 that was recommended by WHO to be included also into the 2017/18 vaccine. However, viruses belonging to a specific 3C.2a1 subcluster was extensively circulating in northern Greece and among vaccinated individuals. Both substitutions carried by this strain were located on antigenic sites and caused losses of N-linked glycosylation sites of the virus, which could potentially affect viral antigenicity. Further studies are needed to determine the antigenicity of this variant strain and its possible implication in vaccine effectiveness.

Genome-wide associations of CD46 and IFI44L genetic variants with neutralizing antibody response to measles vaccine.

Population-based studies have revealed 2-10% measles vaccine failure rate even after two vaccine doses. While the mechanisms behind this remain unknown, we hypothesized that host genetic factors are likely to be involved. We performed a genome-wide association study of measles specific neutralizing antibody and IFNγ ELISPOT response in a combined sample of 2872 subjects. We identified two distinct chromosome 1 regions (previously associated with MMR-related febrile seizures), associated with vaccine-induced measles neutralizing antibody titers. The 1q32 region contained 20 significant SNPs in/around the measles virus receptor-encoding CD46 gene, including the intronic rs2724384 (p value=2.64×10-09) and rs2724374 (p value=3.16×10-09) SNPs. The 1q31.1 region contained nine significant SNPs in/around IFI44L, including the intronic rs1333973 (p value=1.41×10-10) and the missense rs273259 (His73Arg, p value=2.87×10-10) SNPs. Analysis of differential exon usage with mRNA-Seq data and RT-PCR suggests the involvement of rs2724374 minor G allele in the CD46 STP region exon B skipping, resulting in shorter CD46 isoforms. Our study reveals common CD46 and IFI44L SNPs associated with measles-specific humoral immunity, and highlights the importance of alternative splicing/virus cellular receptor isoform usage as a mechanism explaining inter-individual variation in immune response after live measles vaccine.

Rabies Vaccination: Higher Failure Rates in Imported Dogs than in those Vaccinated in Italy.

The current European Union (EU) legislation decrees that pets entering the EU from a rabies-infected third country have to obtain a satisfactory virus-neutralizing antibody level, while those moving within the EU require only rabies vaccination as the risk of moving a rabid pet within the EU is considered negligible. A number of factors driving individual variations in dog vaccine response have been previously reported, including a high rate of vaccine failure in puppies, especially those subject to commercial transport. A total of 21001 observations collected from dogs (2006-2012) vaccinated in compliance with the current EU regulations were statistically analysed to assess the effect of different risk factors related to rabies vaccine efficacy. Within this framework, we were able to compare the vaccination failure rate in a group of dogs entering the Italian border from EU and non-EU countries to those vaccinated in Italy prior to international travel. Our analysis identified that cross-breeds and two breed categories showed high vaccine success rates, while Beagles and Boxers were the least likely to show a successful response to vaccination (88.82% and 90.32%, respectively). Our analysis revealed diverse performances among the commercially available vaccines, in terms of serological peak windows, and marked differences according to geographical area. Of note, we found a higher vaccine failure rate in imported dogs (13.15%) than in those vaccinated in Italy (5.89%). Our findings suggest that the choice of vaccine may influence the likelihood of an animal achieving a protective serological level and that time from vaccination to sampling should be considered when interpreting serological results. A higher vaccine failure in imported compared to Italian dogs highlights the key role that border controls still have in assessing the full compliance of pet movements with EU legislation to minimize the risk of rabies being reintroduced into a disease-free area.

High rate of vaccine failure after administration of acellular pertussis vaccine pre- and post-liver transplantation in children at a children's hospital in Japan.

We assessed the serological response to pertussis vaccines administered pre- and post-liver transplantation in 58 pediatric patients at a children's hospital in Japan. A high rate of pertussis vaccine failure was observed, 44.8% against the pertussis toxin and 69.0% against filamentous hemagglutinin, with no difference in the seropositivity rate with respect to the timing of the vaccination during the peritransplant period.

The impact of vaccine failure rate on epidemic dynamics in responsive networks.

An SIS model based on the microscopic Markov-chain approximation is considered in this paper. It is assumed that the individual vaccination behavior depends on the contact awareness, local and global information of an epidemic. To better simulate the real situation, the vaccine failure rate is also taken into consideration. Our main conclusions are given in the following. First, we show that if the vaccine failure rate α is zero, then the epidemic eventually dies out regardless of what the network structure is or how large the effective spreading rate and the immunization response rates of an epidemic are. Second, we show that for any positive α, there exists a positive epidemic threshold depending on an adjusted network structure, which is only determined by the structure of the original network, the positive vaccine failure rate and the immunization response rate for contact awareness. Moreover, the epidemic threshold increases with respect to the strength of the immunization response rate for contact awareness. Finally, if the vaccine failure rate and the immunization response rate for contact awareness are positive, then there exists a critical vaccine failure rate αc > 0 so that the disease free equilibrium (DFE) is stable (resp., unstable) if α < αc (resp., α > αc). Numerical simulations to see the effectiveness of our theoretical results are also provided.

P0578 : HBsAg negative anti-HBc positive serology increases the risk of HCC in chronic hepatitis patients: A meta-analysis

Background and Aims: There are concerns with regards to the efficacy of combination (DTaP-IPV-HiB-HBV) vaccine, especially in infants with high risk of vertical transmission, and there has been no study comparing the vaccine efficacy in those receiving three doses of monovalent HBV vaccine versus two doses monovalent plus one dose of combination vaccine in infants of chronic HBVinfected mothers. The aim of this study is to examine the anti-HBs antibody levels of both vaccine regimens in a cohort of these highrisk infants. Methods: An observational cohort of term neonates was recruited from June 2009 to June 2014 from multiple primary to tertiary centres in Singapore. Anti-HBs antibody levels were measured at 9 months of age. Vaccine regimen administered were based on the individual institutions’ practice. Results: All term neonates received monovalent hepatitis B vaccine with hepatitis B immunoglobulin at birth with a repeat dose of monovalent hepatitis B vaccine at 1 month. They received either monovalent (n = 111) or combination vaccine (n =95) at 6 months. Baseline maternal age (31.3 year vs 32.9 years), hepatitis B e-antigen positivity (24% vs 28%, p = 0.78) and birth characteristics, including gestational age (38 weeks versus 38weeks, p = 0.34), birth weight (3.13 kg vs 3.16 kg, p = 0.62) were similar between groups. The mean anti-HBs antibody were both not statistically different (478 IU/L vs 469 IU/L, p = 0.88). Vaccine failure rate (anti-HBs antibody <10 IU/L) was 4.5% vs 4.7% (p =1.00). Vertical transmission rate (HBsAg or HBVDNA positive) was 1.8% vs 2.3% (p =0.78). Conclusions: For term neonates delivered by chronic HBVinfected mothers, both 3-dose monovalent and combination vaccine regimen showed similar vaccine efficacy, with no increased risk of vertical transmission.

Can vaccine legacy explain the British pertussis resurgence?

Pertussis incidence has been rising in some countries, including the UK, despite sustained high vaccine coverage. We questioned whether it is possible to explain the resurgence without recourse to complex hypotheses about pathogen evolution, subclinical infections, or trends in surveillance efficiency. In particular, we investigated the possibility that the resurgence is a consequence of the legacy of incomplete pediatric immunization, in the context of cohort structure and age-dependent transmission. We constructed a model of pertussis transmission in England and Wales based on data on age-specific contact rates and historical vaccine coverage estimates. We evaluated the agreement between model-predicted and observed patterns of age-specific pertussis incidence under a variety of assumptions regarding the duration of immunity. Under the assumption that infection-derived immunity is complete and lifelong, and regardless of the duration of vaccine-induced immunity, the model consistently predicts a resurgence of pertussis incidence comparable to that which has been observed. Interestingly, no resurgence is predicted when infection- and vaccine-derived immunities wane at the same rate. These results were qualitatively insensitive to rates of primary vaccine failure. We conclude that the alarming resurgence of pertussis among adults and adolescents in Britain and elsewhere may simply be a legacy of historically inadequate coverage employing imperfect vaccines. Indeed, we argue that the absence of resurgence at this late date would be more surprising. Our analysis shows that careful accounting for age dependence in contact rates and susceptibility is prerequisite to the identification of which features of pertussis epidemiology want additional explanation.

Long‐term efficacy of the hepatitis B Vaccine in a high‐risk group

Chronic infection with hepatitis B virus (HBV) is a global health problem. In an attempt to control infection, worldwide HBV vaccination programs have been established. Saudi Arabia, an endemic area for HBV infection, established an HBV immunization program in 1989. This cross-sectional study evaluates the long-term protection of HBV vaccination 14-24 years after primary immunization in a high-risk group (clinical year medical students) at King Abdulaziz University Hospital in Jeddah, Saudi Arabia. All participants had complete HBV immunization at birth or in early childhood. Hepatitis B surface antibody (anti-HBs) levels were obtained. An anti-HBs titer of <10 mIU/ml indicated no protection, while a titer of >10 mIU/ml was considered to represent protective immune status. A total of 238 students were included; they were predominantly females (n = 182, 76.5%). Mean age was 22.2 ± 1.1 years. Duration since primary vaccination was 19.8 ± 2.3 years. Female students were more likely to maintain long-term protection compared to males (62.1% and 58.8%, respectively). Anti-HBs levels were significantly low in many students after primary immunization. Testing medical students for anti-HBs levels may be warranted as they represent a high-risk population. The higher rate of vaccine failure in males than females requires further investigation as it may explain the higher prevalence of HBV in the male population.

Primary versus secondary failure after varicella vaccination: implications for interval between 2 doses.

Two-dose varicella vaccination is recommended for optimal control of varicella in populations with high (>90%) 1-dose coverage. Optimal timing of the second dose may depend on whether breakthrough varicella results from primary vaccine failure (no protective immunity after vaccination) or secondary vaccine failure (waning protective immunity). Published literature (1995 to 2012) on vaccine failure after varicella vaccination cited in PubMed and other online sources was reviewed. Nineteen publications detailed 21 varicella outbreaks with breakthrough varicella rates ranging from 0% to 42%; the publications showed no consistent trend between breakthrough varicella rate and time since vaccination. Literature to date indicates a relatively high rate of primary vaccine failure and limited evidence of secondary vaccine failure among 1-dose varicella vaccine recipients, suggesting that a short interval between 2 doses might be preferable in countries considering implementation of universal varicella vaccination to reduce breakthrough varicella. However, any potential disruption to well-established vaccination schedules should be considered.