Recovered Covid-19 patients have a lower risk of future infection from the SARS-Cov-2 virus for at least a few months, according to new findings from the National Cancer Institute.
The study released Wednesday offers some insight into how much immunity stems from antibodies produced by a natural infection. The findings could guide critical public health decisions, such as when it’s okay to return to the office and how to decide vaccine priority, the NCI said.
“People who are antibody positive have partial protection against getting a new infection,” Douglas R. Lowy, study co-author and the NCI’s principal deputy director, said in an interview. “When you have a 10-fold difference between two groups— and it’s a very large study the way this is—it’s highly unlikely that the difference is explained because of confounding issues.”
Smaller anecdotal studies have indicated similar protections. The NCI observational study, which appeared Wednesday in JAMA Internal Medicine, used a much larger sample size of real-world data by screening more than 3.2 million patients.
“We hope that these results, in combination with those of other studies, will inform future public health efforts and help in setting policy,” NCI Director Norman E. “Ned” Sharpless and study co-author said in a statement.
However, the study authors noted that “protection induced by a safe vaccine is clearly preferable, as the population wide risk of a serious outcome from an authorized or approved vaccine is expected to be orders of magnitude lower than that from natural infection.”
The Centers for Disease Control and Prevention recommends vaccination regardless of whether someone already had Covid-19.
“When you have a legitimate opportunity to get vaccinated, you should take advantage of it, whether you’re antibody positive, or antibody negative,” Lowy said.
There are three main types of tests to screen for SARS-Cov-2, the virus that causes Covid-19. Molecular, or PCR, tests are highly accurate diagnostics that indicate whether the patient currently has an infection. Antigen tests are more rapid diagnostics but they tend to be less accurate than PCR tests.
Serology tests look for antibodies in part of the blood. The presence of those antibodies can indicate whether someone was previously exposed to the virus but doesn’t necessarily mean they had an infection when they provided the blood sample.
NCI researchers worked with health technology companies HealthVerity and Aetion to cull information from more than 3.2 million patients who took serology tests, including from commercial laboratories such as Quest Diagnostics and Labcorp., electronic medical records, and private insurers.
Stripping those cases of identifying information, the study then looked at how many of those patients followed up with molecular tests.
Researchers found patients who tested positive for antibodies were about a tenth less likely to test positive for the virus three months later, indicating the antibodies provide some degree of protection. A patient who has recovered from the virus typically is no longer infectious after three months, so a positive PCR test after that time would likely mean a reinfection.
“The data from this study suggest that people who have a positive result from a commercial antibody test appear to have substantial immunity to SARS-CoV-2, which means they may be at lower risk for future infection,” lead study author Lynne Penberthy, who is the associate director of NCI’s Surveillance Research Program, said in a statement.
More Studies Needed
Penberthy acknowledged additional studies must be done to understand how long this protection lasts, who may have limited protection, and how other underlying health conditions may affect protections.
The initial rollout of antibody tests last spring was rocky, which raised questions about their accuracy.
The NCI, which runs an advanced serology lab for ongoing work with HPV and other cancers, shifted its work at the Federal National Laboratory last spring to help the Food and Drug Administration evaluate serology tests for SARS-Cov-2. The NCI is also building a national network of antibody testing centers.
This latest study by NCI signals that commercial antibody tests are “pretty reliable,” Lowy said. “Otherwise, you would never see this big difference between the positives and the negatives, if there were a lot of false positives and false negatives.”