Throughout the pandemic, the prevailing thought was that if you got COVID-19, you would have immunity from the virus for around 90 days. But lately, more people have been reporting COVID symptoms more than once in the same month.
So how long does immunity really last, and can you get COVID twice in one month? We asked two top Boston doctors those questions and more during NBC10 Boston's latest "COVID Q&A" discussion.
'A really challenging question'
"That's a really challenging question to answer," Dr. Daniel Kuritzkes of Brigham and Women's Hospital said. "You have to distinguish who has persisting COVID-19. We know there are many people for whom the virus can continue to be detectable. They can continue to have symptoms or seem to improve and then worsen and then improve as opposed to truly becoming re-infected."
"The only way to know for sure that someone has been re-infected is to compare the virus they had originally with the virus they had the second time and show that those are different viruses from a molecular sequencing perspective," he said.
But that isn't happening, Dr. Shira Doron of Tufts Medical Center said, because hospitals aren't saving swabs more than a few days because they don't have room to store them.
"You can't go back to that person's test from a month ago and sequence it to say it's the same or different," she pointed out.
How long does immunity last?
"Most of the epidemiological data would suggest people are generally protected for a couple months," Kuritzkes said. "But there's a caveat to that as well. If you're exposed to a completely different strain, then that new second infection could happen sooner than a few months."
"Looking at it from a big picture perspective, if you look at the data from South Africa, there is a regular periodicity of a new wave coming every four months or so that suggests probable protection for several months, then a new variant emerges and even if you've been infected before, you're susceptible to the new variant," he added.
Did I get COVID twice, or did it just never leave?
"PCR tests stay positive for a very long time -- 28 days, sometimes three or even four months. So that isn't a way to tell if you have a new infection," Doron said. "The other thing is, there is an underappreciation for how common false positives are, even with PCR tests."
Home antigen tests have 11% false positivity, she said, while with PCRs experts don't really know what the exact false positivity rate is.
"Anytime I have an expected new positive off someone who was positive a couple months ago I will repeat it," Doron said. "Most of the time the repeat is negative. So it is really hard. There's a lot of chatter out there about reinfection, but I think it's really hard to prove definitively whether two infections close together in the same person are or are not a new infection."