1. How many people have been reinfected?
A tracker maintained by the Dutch news agency BNO News had recorded 24 cases globally as of Oct. 16. The first confirmed case, a 33-year-old man from Hong Kong, was reported in August. He’d tested positive in March with mild symptoms of Covid-19, the disease caused by SARS-CoV-2, and had two negative tests a few weeks later. Four and a half months after the first event, he tested positive again, although he had no symptoms. The only known person to have died from a case of reinfection was an 89-year-old Dutch woman, who was also undergoing chemotherapy treatment for a rare white blood cell cancer.
2. How is a case of reinfection distinguished from a single case of prolonged illness?
To demonstrate reinfection, scientists have to isolate the microbial culprit each time, check its genetic fingerprint, and show that each infection was caused by a different strain of SARS-CoV-2.
3. Is reinfection with a virus unusual?
Not at all. Only some viruses, such as the one that causes measles, provide their victims with something close to lifelong protection against a second bout. Common viruses which, like SARS-CoV-2, cause respiratory disease — such as those responsible for colds and flu — are prevalent largely because of their ability to reinfect. A number of factors can allow for repeat infection, including an insufficient response by the immune system, waning immunity, and a mutation of the virus to the extent that people are essentially encountering the latest version for the first time.
4. What’s the reason with SARS-CoV-2?
It’s not clear, although the virus’s mutation rate has so far not raised big alarms. There are clues that infection with it does provide some immunity for some time. In one study, Rhesus macaques infected with the virus appeared to be protected against reinfection when exposed to an identical strain in the early recovery phase. A number of studies have concluded that people produce antibodies for up to 7 months after infection. However, it’s not clear that their presence is sufficient to prevent reinfection, and research has also shown that antibody levels fall off rapidly, especially for those with mild cases. In the majority of reinfections tracked by BNO News, patients had mild or no symptoms in the first instance. The time between their infections was as short as 12 days. The average was 74 days, casting doubt on U.S. President Donald Trump’s assertion, after his own bout with Covid-19, that he was “immune” for at least four months.
5. What happens in the second infection?
In 10 of the 19 cases BNO News has tracked where full details are known, the second infection produced worse symptoms than the first, and in five instances it produced serious illness. Researchers think one reason could be that on the second occasion the patient was exposed to a higher or more virulent dose of the virus. It’s unknown whether those who are reinfected can transmit the virus to others. In light of the confirmed reinfections, a group of researchers recommended in the Lancet that those who’ve had Covid-19 should take the same precautions as everyone else against it.
6. What do the reinfections say about prospects for an effective vaccine?
Not necessarily anything. In the one reported case of reinfection where the first bout was known to be serious, the second was mild. One theory is that the immune system requires a tough challenge in order to be adequately prepared to take on the virus. A vaccine might provide that training, and not necessarily just by producing antibodies. Several experimental Covid-19 vaccines prompt the production of a less well-known immune player, T cells. While preventing infection with SARS-CoV-2 is the ultimate goal, researchers say that a vaccine that protects against illness or even against severe disease would be very useful.