Few health professionals in the UK who recovered from COVID-19 and had immunoglobulin G (IgG) antibodies to the virus were reinfected in the next 6 months, according to a study published on December 23 in New England Journal of Medicine.
The prospective longitudinal cohort study involved measuring the levels of IgG antibodies against the peak protein of the coronavirus and the nucleocapsid in symptomatic and asymptomatic health professionals in Oxford University hospitals submitted to the COVID-19 test. Testing began on March 27 and follow-up ended on November 30.
In screening, 11,364 team members were identified as having 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 who tested negative for anti-spike antibodies and positive for COVID-19 at the initial screening, 100 were asymptomatic and 123 had symptoms.
Similar rates of reinfection with both types of antibodies
Of the 1,265 employees who had antibodies, only two tested positive for COVID-19 at the start of the study; none had symptoms. But three 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 the antibody test; after five negative COVID-19 tests, the worker tested positive on day 190, but without symptoms and subsequently tested negative and had no increase in antibody levels. A fourth team member with both types of antibodies tested positive for COVID-19 231 days after an initial infection, but was negative on two subsequent tests; subsequent antibody assays demonstrated decreasing levels of both types of antibodies.
Another 864 with antibodies (68%) remembered having characteristic symptoms of COVID-19 in the past, while 466 (37%) had a confirmed previous SARS-CoV-2 infection (262 with symptoms).
Of the 11,364 workers without coronavirus antibodies, 2,860 (25%) remembered having symptoms of COVID-19 prior to screening, and 24 (0.2%) had tested positive for infection (all of the latter were asymptomatic).
After adjusting for age, sex and month of screening or calendar time as a continuous variable, the ratio of the incidence rate in team members with anti-spike antibodies was 0.11, and the positive COVID-19 results were inversely associated with anti-spike antibody tests— regardless of whether they are above or below the positive limit (P<0.001 for trend).
Likewise, of the 12,666 employees in whom the anti-nucleocapsid IgG was used as a marker of previous COVID-19 infection, 226 of 11,543 workers without the antibodies tested positive for COVID-19, versus 2 of 1,172 workers with antibodies (rate incident adjusted proportion, 0.11). But the rate of positive results from the COVID-19 test dropped with increasing levels of anti-nucleocapsid antibody titers (P<0.001 for trend).
Overall, 12,479 healthcare professionals had anti-peak and anti-nucleocapsid antibodies at baseline. Of the 11,182 negative employees for both types of antibodies at the start of the study, 218 subsequently tested positive for COVID-19, against 1 out of 1,021 positive workers for both (incident rate ratio, 0.06) and 2 out of 344 with results of mixed antibody test (incident rate ratio, 0.42).
Immunity requires additional characterization
The authors noted that the presence of anti-peak 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 workers with positive antibodies, both asymptomatic. ” which suggests that the previous infection that resulted in antibodies to SARS-CoV-2 is associated with protection against reinfection for most people for at least 6 months, “they said.
The researchers said they cannot conclude whether previous positive antibody results or current levels determine immunity or whether protection is conferred by the measured antibodies or by the protection of T cells, which has not been evaluated. They asked for future studies in children, older people and people with underlying medical conditions, such as immunosuppression.
“Continuous follow-up is required in this and other cohorts, including the use of SARS-CoV-2 humoral and cellular immunity markers, to assess the magnitude and duration of protection against reinfection, symptomatic disease and hospitalization or death and the effect of protection in transmission “, wrote the authors.