December 29, 2020

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

The presence of anti-spike or anti-nucleocapsid IgG antibodies was associated with a “substantially” reduced risk of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) reinfection in the ensuing 6 months, according to a study published in The New England Journal of Medicine

“No symptomatic infections and only two polymerase chain reaction (PCR)-positive results in asymptomatic health care workers were seen in those with anti-spike antibodies, which suggests that previous infection resulting in antibodies to SARS-CoV-2 is associated with protection from reinfection for most people for at least 6 months,” reported Sheila F Lumley, Oxford University Hospitals NHS Foundation Trust, United Kingdom, and colleagues. 

In this longitudinal cohort study, researchers investigated the incidence of SARS-CoV-2 infection in seropositive and seronegative health care workers attending testing of asymptomatic and symptomatic staff at Oxford University Hospitals in the United Kingdom. Baseline antibody status was determined by anti-spike (primary analysis) and anti-nucleocapsid IgG assays. Testing began on March 27, 2020 and staff members were followed until November 30, 2020. 

A total of 12,541 health care workers underwent measurement of baseline anti-spike antibodies. Of these, 11,364 (90.6%) were seronegative and 1,177 (9.4%) were seropositive at their first anti-spike IgG assay, with seroconversion occurring in 88 workers during the study. Health care workers were followed for a median of 200 days after a negative antibody test and for 139 days after a positive antibody test.

A total of 223 anti-spike-seronegative health care workers had a positive PCR test (1.09 per 10,000 days at risk) whereby 100 were tested during screening while they were asymptomatic and 123 while symptomatic. Meanwhile, 2 anti-spike–seropositive health care workers had a positive PCR test (0.13 per 10,000 days at risk), and both workers were asymptomatic when tested (adjusted incidence rate ratio, 0.11; 95% confidence interval [CI], 0.03-0.44; P = 0.002). 

With anti-nucleocapsid IgG used as a marker for prior infection in 12,666 health care workers, 226 of 11,543 (1.10 per 10,000 days at risk) seronegative health care workers tested PCR-positive, compared with 2 of 1172 (0.13 per 10,000 days at risk) antibody-positive health care workers (incidence rate ratio adjusted for calendar time, age, and gender, 0.11; 95% CI, 0.03-0.45; P = 0.002).

Further, a total of 12,479 health care workers had both anti-spike and anti-nucleocapsid baseline results. Of 11,182 workers with both immunoassays negative, 218 had subsequent PCR-positive tests (1.08 per 10,000 days at risk). Meanwhile, subsequent PCR-positive tests occurred in 1 of 1,021 workers (0.07 per 10,000 days at risk) with both baseline assays positive (incidence rate ratio, 0.06; 95% CI, 0.01-0.46) and 2 of 344 workers (0.49 per 10,000 days at risk) with mixed antibody assay results (incidence rate ratio, 0.42; 95% CI, 0.10-1.69). 

“In this study, we found a substantially lower risk of reinfection with SARS-CoV-2 in the short term among health care workers with anti-spike antibodies and those with anti-nucleocapsid antibodies than among those who were seronegative,” the authors concluded. However, the authors noted that they were unable to conclude whether protection was conferred through the antibodies measured or through T-cell immunity, which was not assessed in the study.

“Our study was relatively short, with up to 31 weeks of follow-up. Ongoing follow-up is needed in this and other cohorts, including the use of markers of both humoral and cellular immunity to SARS-CoV-2, to assess the magnitude and duration of protection from reinfection, symptomatic disease, and hospitalization or death and the effect of protection on transmission,” the authors added. 

In addition, the authors said further studies are needed to assess postinfection immunity in other populations, including children, older adults, and persons with coexisting conditions, including immunosuppression, as this study was of predominantly healthy adult health care workers 65 years of age or younger.

SOURCE: The New England Journal of Medicine
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