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Antibody status and incidence of SARS-CoV-2 infection in health care workers

Our take —

In a cohort of 12,541 health care workers in the United Kingdom, the risk of re-infection among those who had a positive antibody test was found to be low: 1.09 per 10,000 days at risk. The rate of infection after baseline was significantly and substantially lower (reduced by 89%) among those with antibodies at baseline compared to those without antibodies at baseline. These findings, while promising, represent data for up to 31 weeks of follow-up and for a relatively healthy population. Further research is needed to examine longer lasting potential antibody protection in higher-risk populations.

Study design

Prospective Cohort

Study population and setting

A prospective longitudinal cohort was created among 12,541 health care workers at Oxford University Hospitals in the UK. The goal of this cohort was to assess levels of infection after baseline among those who were seropositive (had SARS-CoV-2 antibodies) and those who were seronegative (did not have SARS-CoV-2 antibodies) at baseline. Antibody status at baseline was determined by presence of anti-spike and anti-nucleocapsid IgG assays. Infection after baseline was determined by a positive SARS-CoV-2 PCR test. Initial PCR testing for symptomatic staff began March 27, 2020 and testing every two weeks for asymptomatic staff began April 23, 2020. The end of the study period was November 30, 2020. Overall, staff were followed for up to 31 weeks.

Summary of Main Findings

Among the 12,541 health care workers in the cohort, 11,363 were seronegative (did not have SARS-CoV-2 antibodies) and 1265 were seropositive (did have SARS-CoV-2 antibodies) at baseline. Among those who were seronegative, 223 (100 asymptomatic and 123 symptomatic) had a positive PCR test during follow-up (1.09 per 10,000 days at risk). Among those who were seropositive, 2 had a positive PCR test during follow-up (0.13 per 10,000 days at risk) and both were asymptomatic. The relative incidence of positive PCR comparing those who were seropositive to seronegative at baseline was 0.11 (95% CI: 0.03 to 0.44), suggesting that presence of antibodies is associated with significantly reduced incidence of subsequent infection (for up to approximately 31 weeks).

Study Strengths

Because regular PCR testing was conducted in this group of health-care workers, regardless of symptom status, it was possible to create a cohort of individuals in which to study differences in prospective infection by presence of antibodies.


Follow-up of this cohort was limited to 31 weeks, and less can be said from these results about levels of reinfection for longer time periods. Asymptomatic screening was offered but conducted on a voluntary basis, and so it is possible that asymptomatic infection was not always captured. This may have been especially true in the seropositive group, as those who knew they were seropositive may have been even less likely to attend screening testing. The cohort was relatively homogenous (<65 years old, healthy).

Value added

This study helps shed light on the relatively low incidence of SARS-CoV-2 re-infection among those with antibodies for up to 6 months.

This review was posted on: 26 March 2021