The first confirmed case of Covid-19 reinfection in the US has added to doubts about “herd immunity” from the virus, and worried experts because the patient became more seriously ill following the second infection.
A Nevada man tested positive for Sars-Cov-2, the virus responsible for Covid-19, after developing moderate symptoms in April. He then suffered more severely two and a half months later, requiring emergency oxygen therapy.
University of Nevada scientists who examined the 25-year-old over the period found many more differences in the two genetic fingerprints than could be explained by mutations during one long illness, confirming two separate infections.
“Most of us have thought that reinfection with Covid-19 was likely to become common as individuals’ immunity levels declined post infection,” commented Paul Hunter, professor of medicine at University of East Anglia.
However, the Nevada case was “very concerning both from the point of view of the very short time between the two infections and the fact that the second illness was more severe than the first”, he added. The study was published in Lancet Infectious Diseases.
Coronavirus reinfections have been recorded in Hong Kong, Belgium, the Netherlands and Ecuador but this is only the second time that a patient has showed more serious symptoms when the virus returned.
The University of Nevada authors presented several possible explanations for the severity of the reinfection. The patient might have encountered a very high dose of the virus on the second occasion, which caused a more acute reaction. Or he might have come in to contact with a more virulent viral strain, though scientists have not yet noted mutations in Sars-Cov-2 associated with more serious disease.
Another explanation would be an unusual phenomenon known as antibody dependent enhancement (ADE). In this counter-intuitive and potentially dangerous condition, the presence of antibodies, which are supposed to fight disease, actually make a subsequent infection worse. ADE has been observed previously in the closely related coronavirus disease Sars in 2003 and in viral diseases such as dengue fever but not so far in Covid-19.
“We need more research to understand how long immunity may last for people exposed to Sars-Cov-2 and why some of these second infections, while rare, are presenting as more severe,” said Mark Pandori, lead author of the Nevada study. “So far, we’ve only seen a handful of reinfection cases, but that doesn’t mean there aren’t more, especially as many cases of Covid-19 are asymptomatic. Right now, we can only speculate about the cause of reinfection.”
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The implications of the growing list of confirmed Covid-19 reinfections for vaccine development are far from clear, scientists say.
“As more cases of reinfection surface, the scientific community will have the opportunity to understand better the correlates of protection and how frequently natural infections with Sars-Cov-2 induce that level of immunity,” said Akiko Iwasaki, professor of immunobiology at Yale University. “This information is key to understanding which vaccines are capable of crossing that threshold to confer individual and herd immunity.”
She added: “Reinfection cases tell us that we cannot rely on immunity acquired by natural infection to confer herd immunity. Not only is this strategy lethal for many but also it is not effective. Herd immunity requires safe and effective vaccines.”