Across the US, Americans are anxious to learn whether they may have been infected with the coronavirus – which they hope would offer some immunity going forward. In New York, researchers found that nearly 20% of the population tested positive for antibodies.
But while the tests may be helpful in understanding how the virus moves through a population, researchers say much remains unknown: not all tests produce consistently accurate results, and scientists are still researching whether and for how long antibodies offer immunity from reinfection. Bottom line: even if you test positive for antibodies, experts urge social distancing and other precautionary measures.
What can the presence of antibodies tell us about immunity from Covid-19?
Elitza Theel: We are continuing to learn about that. Preliminary studies in rhesus macaque monkeys show that after they’re infected with Sars-Cov-2 [the virus that causes Covid-19] , if [researchers] attempt to reinfect those same animals about a month later, the animals have high antibody levels and don’t develop symptoms.
This hasn’t been shown in humans yet, but we do probably have at least some partial protective immunity for some duration of time [after infection]. But we don’t know what level of antibodies you need to be protected or how long those antibodies last.
How long would it take to learn how long protective immunity lasts?
Akiko Iwasaki: Maybe not too long because, unfortunately, there’s increasing Covid-19 incidence in the south. If [people from the south] travel or if people from the north travel to the south and acquire the infection, there will be a second wave in the north-east. And if that happens, then we should be in a position to analyze these second waves and second exposures.
How accurate are antibody tests?
Theel: That’s a hard question because there are so many different antibody tests out there. We’ve looked at over 15 tests, and some of them performed very well, meaning that their specificity is above 99% – less than 1% false-positive results. Other tests that have false-positive rates of 5 to 15% which is concerning.
Iwasaki: The FDA is considering many of these tests for approval, so hopefully, we’ll have multiple validated kits out there, but I wouldn’t say every [test] out there right now is accurate.
What’s the benefit of getting tested for antibodies at this point – at both an individual level and at the population level?
Theel: On the individual level, the recommendation is to not change any social distancing or other practices based on an antibody test results, simply because we don’t know how long or even if there is protective immunity against reinfection if you’re antibody-positive. But you could be a potential convalescent plasma donor. Down the road as vaccines hopefully [are distributed], antibody testing will be important to monitor the immune response of individuals who are part of clinical trials.
Iwasaki: At the population level it’s much more informative. It tells us what percent of the population has been exposed to the virus, regardless of whether they experienced symptoms. I suspect there are lots of asymptomatic or mild cases where people were actually exposed to Sars-Cov-2, and those people, hopefully, develop a detectable level of antibody.
What role can antibody tests play in reopening? Can those with antibodies be issued immunity passports to allow them to go back to work or travel?
Iwasaki: I’m skeptical about immunity passports in general because we don’t know what level of antibodies are needed for protection.
Do you think there’s a chance the pandemic burns out due to herd immunity before a vaccine is developed?
Iwasaki: Even though the US has the most number of cases in the world, it’s still a small fraction of the entire population. For natural infection to spread and confer herd immunity, it would first kill millions of people.
Theel: For herd immunity, we need a significant proportion – 50 to 70% – of the population to be infected. Even though we’re seeing a striking increase in cases recently, we’re nowhere near that threshold. I think we’ll have a vaccine before we reach a natural herd immunity state.
How optimistic are you about an effective vaccine being developed?
Theel: I’m optimistic. From history, we know that the first version of a vaccine may not be the best. There are usually multiple different formulations of a vaccine before the best one is achieved. But even those initial vaccines still provide some protection.
Elitza Theel, director of the infectious diseases serology laboratory at Mayo Clinic
Akiko Iwasaki, professor of immunology at Yale University