In recent weeks, the first confirmed reports of people who have been reinfected with the coronavirus have begun to trickle in. Such cases suggest that, in some people at least, the immune system doesn’t develop lasting protection against the virus. How worried should we be?
Immunologists weren’t surprised that there were cases of reinfection, as we see this all the time with many viruses, such as seasonal strains of the flu. This may be because the defences raised by our immune systems for one form of a virus don’t work as well for another.
Long-lasting immunity is thought to be generated by memory immune cells. Some of these cells produce antibodies – molecules that can either prevent a future infection from occurring or stop a second infection from causing disease. Memory T-cells, on the other hand, guide the immune response or kill infected cells.
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So far, and suggests that many people will have antibodies against the coronavirus for months. So why do some people appear to have caught it twice? Immunity might not last for everyone, while in other cases, it is possible that the initial immune response wasn’t strong enough in the first place.
Some immunologists predict that the current coronavirus is likely to trigger immunity in a similar way to the virus behind the SARS outbreak in 2003, as the two viruses are genetically similar. The SARS virus typically induces a long-lasting immunity, says Petter Brodin at the Karolinska Institute in Stockholm. One recent study found that .
Yet John Brooks at the US Centers for Disease Control cautions against assuming that the viruses act in the same way. “The diseases they produce and the way they are being transmitted are remarkably different,” he says.
The first confirmed case of reinfection of the coronavirus is thought to be that of a 33-year-old man in Hong Kong, who experienced mild symptoms when he first tested positive. Four and a half months later, he tested positive again, despite having no symptoms. Genetic analyses of the two virus samples suggest they were caused by different forms of the virus, indicating they were indeed two separate infections.
This case was somewhat reassuring to Brodin, because it indicates that the man’s immune system had developed a lasting response that protected against future disease, if not reinfection. “This is a textbook example of how the immune system should respond,” he says.
Another reported case of reinfection in  is more worrying. The man first fell ill with covid-19 in April. He recovered and tested negative for the virus twice, but 48 days after initially testing positive, the man tested positive again. The virus appeared genetically different to the one from his first infection, but his symptoms were worse the second time around.
This could be “really bad news”, says Brodin, because it suggests that the immune system’s first response could make the disease worse the second time around, through a mechanism known as antibody-mediated enhancement of disease. In this process, the virus can essentially hijack antibodies that have been trained to recognise it, then use them to enter more cells in the body.
It is also possible that the man in Nevada’s second positive result was due to his continued shedding of non-infectious viral RNA from his initial infection, says Brooks. There is a chance that a person can become infected with two forms of a virus at the same time, so the man’s symptoms could have been caused by an entirely different virus, he says.
supports this theory. Researchers found that none of the 285 people they studied who appeared to have been reinfected with the coronavirus passed on the infection to any of their contacts. The team behind this report says that, rather than referring to “reinfection”, they will now refer to “re-positive”, clarifying that such individuals merely test positive for the virus a second time.
For now, it isn’t clear how many of the millions of people who have had covid-19 might have caught the virus twice.
“We have no evidence to suggest [reinfections] are anything but very rare,” says Angela Rasmussen at Columbia University Mailman School of Public Health in New York. “Most of the time, your immune system does protect you from disease against subsequent exposures to a given pathogen.”
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