Few healthcare workers in the UK who have recovered from COVID-19 and had immunoglobulin G (IgG) antibodies to the virus have been re-infected over the next six months, according to a study released on December 23 in the New England Journal of Medicine.
The prospective, longitudinal cohort study performed the measurement of levels of IgG antibodies against the coronavirus’ peak protein and nucleocapsid in symptomatic and asymptomatic health workers at Oxford University hospitals undergoing COVID-19 testing. The test began on March 27 and the follow-up ended on November 30.
The screening identified 11,364 staff members who had no antibodies to SARS-CoV-2, the virus that causes COVID-19, while 1,265 tested positive for antibodies, including 88 who tested negative only later.
Of the 223 workers tested negative for antispike antibodies and positive for COVID-19 at first screening, 100 were asymptomatic and 123 had symptoms.
Similar re-infection rates with both antibody types
Of the 1,265 staff members who had antibodies, only two tested positive for COVID-19 at baseline; none had symptoms. But three were tested positive for coronavirus infection 160 to 199 days later, one with anti-spike IgG, one with anti-nucleocapsid IgG and one with both.
The worker with both antibodies was infected with coronavirus before testing for antibodies; after five negative COVID-19 tests, the worker had one positive test on day 190, but no symptoms, and later tested negative and no antibody levels rose. A fourth staff member with both types of antibodies tested positive for COVID-19 231 days after an initial infection, but was negative during two subsequent tests; subsequent antibody tests showed decreasing levels of both types of antibodies.
Another 864 with antibodies (68%) remembered that they had previously had symptoms of COVID-19, while 466 (37%) had a previously confirmed SARS-CoV-2 infection (262 with symptoms).
Of the 11,364 workers without coronavirus antibodies, 2,860 (25%) recalled having COVID-19 symptoms before screening, and 24 (0.2%) had previously tested positive for infection (all of which were asymptomatic). .
After adjusting for age, sex, and month of screening or calendar time as a continuous variable, the prevalence ratio in staff members with antispike antibodies was 0.11, and positive COVID-19 results were inversely associated with antibody tests – whether they were above or below the positive threshold was (P<0.001 for trend).
Similarly, of the 12,666 staff members in whom anti-nucleocapsid IgG was used as a marker of previous COVID-19 infection, 226 out of 11,543 workers without the antibodies tested positive for COVID-19, compared with 2 out of 1172 workers with antibodies ratio, 0.11). However, the rate of positive COVID-19 test results decreased with rising levels of anticleocapsid antibody titers (P<0.001 for trend).
Overall, 12,479 health workers had antibodies as well as anti-nucleocapsid antibodies. Of the 11,182 staff members who were negative for both types of antibodies against the base, 218 later tested positive for COVID-19, compared with 1 in 1,021 workers who were positive for both (incidence rate ratio, 0.06) and 2 of 344 with mixed antibody test results ratio, 0.42).
Immunity requires further characterization
The authors noted that the presence of antispike antibodies was associated with a much lower risk of SARS-CoV-2 infection during follow-up and that only two COVID-19 reinfections occurred in antibody-positive workers, both asymptomatic, resulting in “indicates that previous infection resulting in antibodies to SARS-CoV-2 is associated with protection against most people for at least 6 months against reinfection,” they said.
The researchers said they could not deduce whether positive antibody results or current levels determine immunity and whether the protection is provided by the antibodies measured, or by the protection of T-cell, which has not been evaluated. They called for future studies in children, the elderly and those with underlying medical conditions such as immunosuppression.
Continuous follow-up is needed in these and other groups, including the use of markers of both humoral and cellular immunity against SARS-CoV-2, to determine the extent and duration of protection against reinfection, symptomatic diseases and hospitalization or death and the effect of protection on the transfer, ‘the authors wrote.