Preliminary results from a study of patients who have recovered from COVID-19 found that nearly all participants created antibodies to the virus. Levels of antibody creation did not differ by age or sex, and even patients who exhibited only mild symptoms were able to produce a healthy amount.
Published online to medRxiv, the study measured the antibody levels in plasma samples from 1,343 patients in and around New York City. Enrollees were pooled from patients of the Mount Sinai Health System who had recovered from COVID-19 and signed up to donate convalescent plasma. This plasma collection project has enrolled more than 15,000 people so far, but the study analyzed samples from only the first set of donors. Only 3% of study participants had been hospitalized because of COVID-19; the rest reported mild or moderate symptoms.
Out of the first round of 624 patients, 511 had high levels of antibodies, 42 had low levels, and 71 had no present antibodies. Sixty-four of the participants with low or no antibodies were retested more than a week later and all but three had at least some antibodies, suggesting that the optimal time for testing is well after the onset of symptoms.
Researchers later included another 719 participants who suspected they had contracted the virus but had not been diagnosed. In this group, however, 62% of participants did not have antibodies. These participants may have been tested before antibodies were detectable, but they may also have mistaken influenza or allergies for COVID-19. A previous antibody survey conducted by the state of New York estimated that 20% of city residents had been infected.
The study also found that diagnostic PCR tests can be positive for up to 28 days after the start of infection. Since these tests look for genetic fragments of an active or waning infection, a positive result so long after symptoms appear does not necessarily indicate the presence of infectious virus. Researchers in South Korea recently found that several suspected cases of “reinfection” were a result of PCR tests picking up remnants of dead virus.
Experts say the next step will be to confirm that the presence of antibodies equates to protection from the coronavirus and to determine how long that immunity will last.