Jack Lipton, Ph.D.:
If what we know about viruses applies to the novel coronavirus, then the risk of reinfection is primarily associated with either viral mutations or time since your prior infection.
Think about the flu. You can become reinfected with the flu every year. The flu frequently mutates, or changes*, each season. As a result, your body may not recognize it when it comes back around. Think of your body’s immune system like the face on a “Wanted” poster. Those posters are your immune system’s memory of prior infection, and it keeps information to protect it against a future infection. If the face or description on the Wanted poster doesn’t accurately match the criminal, law enforcement might not notice when they come back into town. Similarly, your body’s immune system may not react to a mutated form of a virus it has already been infected with if it looks too different from before. If this happens with the novel coronavirus, it is possible that people could become reinfected. The flu shot is given every year because of changes in strains and mutations that you may encounter each flu season. Based on the data so far, there have been mutations in the novel coronavirus, but it seems as though those changes have been infrequent.
The other challenge for reinfection is time. Think about those “Wanted” posters again, but this time think about them bleaching or fading away in the sun. Your body produces antibodies to a virus to fight off infection. Over time, your antibody response may slowly wane until your body is not able to mount a sufficient response to prevent a new infection, even for the exact same virus. This is the reason why you have to get booster shots of certain vaccines.
*As it reproduces — makes copies of itself — random errors occur in the copies, and those copies reproduce. The flu virus mutates frequently, so the chances that any of those mutations will be meaningful is higher.
What about the reports of people who tested positive for Covid-19, then negative, then positive again?
Reports of a person testing positive, negative, and positive again are from tests performed on the same person over several weeks. Like everything, we don’t know for sure the reason for this right now, but it’s likely not a reinfection. It seems more likely that it has to do with two factors: the sensitivity of the PCR test and the qualities of the sample.
Imagine that you need a test, and you go to a testing site. The most common form of the SARS-CoV-2 test is done using a nasopharyngeal swab. The person performing the test — let’s say, Bill — swabs the back of your nasal cavity, which drains into the back of your throat. It’s not that comfortable, and you might have to work hard to hold steady for the test. Bill will work as quickly as he can in order to get a sample. Whatever Bill is able to get onto the swab becomes our sample. We use a PCR machine to test that sample for the virus.
When the infection begins, the respiratory tract is filled with viral particles, and it’s very easy for us to detect that with PCR. Let’s pretend your sample is positive for SARS-CoV-2.
As the disease progresses in your body over time, the virus is cleaned out by your immune system, and viral levels decrease. You get another test, this time by Sophía. Just like the first time, you might have to work hard to hold steady for the test, and Sophía will work as quickly as she can in order to get a sample. This time, the viral particles may be fewer on the sample, or maybe the particles in your nasal cavity weren’t in the spot Sophía was able to swab, since they’re no longer filling your respiratory tract like they were at the start of the infection.
A PCR has limits in its ability to detect very small amounts of virus, using the types of samples that are commonly used for this test. So over the course of the many weeks the virus could potentially be detected on a PCR machine, your tests might come back as positive, negative, and positive again. This may look like a re-emergence of infection, but it’s really a combination of what we get on your sample and the limits of a PCR’s ability to detect low levels of virus.
From Johns Hopkins Bloomberg School of Public Health:
Reinfection is always a possibility with a viral infection, particularly if you have a subclinical infection and don’t mount much of an immune response against it. Reinfection is also possible within the window after the first infection and before you develop antibodies. That window can vary from a couple of weeks to a few months, depending on how much your immune system was triggered.
We’ve now got a good population of people who have recovered from the virus. Serum samples from those patients can allow us to time exactly when they begin to develop antibodies and when they develop sufficient titers and neutralizing antibodies. This will help us determine what the window is for protection.
Before your immune system returns to normal, you can be infected by not just this virus, but by regular colds and flu. A couple of months may be a reasonable window of recovery, based on what we know right now.
Rachel Graham, PhD, is an assistant professor in the Department of Epidemiology in the UNC Gillings School of Global Public Health. She began actively studying coronaviruses just prior to the SARS-CoV pandemic in 2002–2003.