Abstract
BackgroundData on breakthrough SARS-CoV-2 Delta variant infections in vaccinated individuals are limited.MethodsWe studied breakthrough infections among Oxford-AstraZeneca vaccinated healthcare workers in an infectious diseases hospital in Vietnam. We collected demographic and clinical data alongside serial PCR testing, measurement of SARS-CoV-2 antibodies, and viral whole-genome sequencing.FindingsBetween 11th–25th June 2021 (7-8 weeks after the second dose), 69 staff tested positive for SARS-CoV-2. 62 participated in the study. Most were asymptomatic or mildly symptomatic and all recovered. Twenty-two complete-genome sequences were obtained; all were Delta variant and were phylogenetically distinct from contemporary viruses obtained from the community or from hospital patients admitted prior to the outbreak. Viral loads inferred from Ct values were 251 times higher than in cases infected with the original strain in March/April 2020. Median time from diagnosis to negative PCR was 21 days (range 8–33). Neutralizing antibodies (expressed as percentage of inhibition) measured after the second vaccine dose, or at diagnosis, were lower in cases than in uninfected, fully vaccinated controls (median (IQR): 69.4 (50.7-89.1) vs. 91.3 (79.6-94.9), p=0.005 and 59.4 (32.5-73.1) vs. 91.1 (77.3-94.2), p=0.002). There was no correlation between vaccine-induced neutralizing antibody levels and peak viral loads or the development of symptoms.InterpretationBreakthrough Delta variant infections following Oxford-AstraZeneca vaccination may cause asymptomatic or mild disease, but are associated with high viral loads, prolonged PCR positivity and low levels of vaccine-induced neutralizing antibodies. Epidemiological and sequence data suggested ongoing transmission had occurred between fully vaccinated individuals.FundingWellcome and NIH/NIAID
Highlights
To date, existing data showed that breakthrough Delta variant infections among vaccinated people had comparable viral loads with those in unvaccinated individuals infected with the Delta variants, but there has been no study comparing viral loads of breakthrough infections with those in cases infected with the original SARS-CoV-2 strains detected in early 2020
As per the national COVID-19 management policy in Vietnam, all the 69 infected members of Hospital for Tropical Diseases (HTD) staff were admitted to HTD for clinical follow-up
We studied Oxford-AstraZeneca vaccine breakthrough infections associated with SARS-CoV-2 Delta variant among staff in a 550-bed infectious diseases hospital in HCMC, Vietnam between 11th and 25th June 2021
Summary
Research in contextEvidence before this studyTo date, existing data showed that breakthrough Delta variant infections among vaccinated people had comparable viral loads with those in unvaccinated individuals infected with the Delta variants, but there has been no study comparing viral loads of breakthrough infections with those in cases infected with the original SARS-CoV-2 strains detected in early 2020. This study examined 62 (asymptomatic (n=13) and mildly symptomatic (n=49)) cases of breakthrough Delta variant infections among Oxford-AstraZeneca vaccinated healthcare workers in an infectious diseases hospital in Ho Chi Minh City, Vietnam, and demonstrated evidence of secondary transmission between vaccinated individuals through the analysis of epidemiological and viral whole-genome sequence data. Peak viral loads assessed by Ct value were 251 times higher than those in cases infected with the original SARS-CoV-2 strain detected in Vietnam between March and April 2020. Vaccineinduced neutralizing antibodies after the second dose and at diagnosis were lower than those in the matched uninfected, fully vaccinated controls, but they were not associated with peak viral loads (i.e. infectivity) or the development of symptoms during the course of infection. Interpretation: Breakthrough Delta variant infections following Oxford-AstraZeneca vaccination may cause asymptomatic or mild disease, but are associated with high viral loads, prolonged PCR positivity and low levels of vaccine-induced neutralizing antibodies.
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