Throughout the pandemic, the prevailing thought has been that if you contract COVID-19, you would be immune to the virus for about 90 days. But lately, more people have reported COVID symptoms more than once in the same month.
So how long does immunity really last and can you get COVID twice in a month? We posed these questions and more to two of Boston’s leading physicians during NBC10 Boston’s latest “COVID Q&A” discussion.
“A really challenging question”
“That’s a really difficult question to answer,” said Dr. Daniel Kuritzkes of Brigham and Women’s Hospital. “You have to distinguish who is suffering from COVID-19. We know that there are many people in whom the virus can still be detectable. They may continue to have symptoms, or they may appear to improve and then get worse and then improve rather than actually getting sick-infected.”
“The only way to know for sure that someone has been reinfected is to compare the virus they had originally with the virus they had the second time and show that from the point of view of the virus it was different molecular sequencing are different viruses,” he said.
But that doesn’t happen, said Dr. Shira Doron of Tufts Medical Center because hospitals don’t keep swabs for more than a few days because they don’t have space to store them.
“You can’t go back to that person’s test from a month ago and sequence it to be the same or different,” she said.
How long does immunity last?
“Most of the epidemiological data suggests that people are generally protected for a couple of months,” Kuritzkes said. “But there is also a caveat. If you are exposed to a completely different strain, this new second infection could appear sooner than a few months.”
“If you look at it from an overall perspective, if you look at the data from South Africa, there is a regular periodicity of a new wave coming about every four months, indicating likely protection for several months, then a new variant emerges and even if you’ve been infected before, you’re vulnerable to the new variant,” he added.
Did I get COVID twice or just never went?
“PCR tests remain positive for a very long time – 28 days, sometimes three or even four months. So that’s not a way to tell if you have a new infection,” Doron said. “The other thing is that there is too little appreciation for how common false positives are, even with PCR tests.”
Home antigen tests have 11% false positivity, she said, while experts on PCRs don’t really know what the exact false positivity rate is.
“Any time I expect a new positive result from someone who was positive a few months ago, I will repeat it,” Doron said. “Most of the time, the replay is negative. So it’s really hard. There’s a lot of talk about reinfection, but I think it’s really hard to definitively prove whether two infections that are close together in the same person are new infections or not.”