Can You Catch Covid-19 More Than Once?

Studies suggest the answer is yes.

A few studies have now confirmed that people can indeed catch the virus twice.

Researchers at the University of Hong Kong’s (HKU) department of microbiology identified a patient in Hong Kong was diagnosed with Covid-19 for the second time – and it wasn’t a continuation of his old illness, rather a new strain altogether.

The “apparently young and healthy patient” had a second episode of Covid-19 infection, which was diagnosed four and a half months after the first. The second time, he had no symptoms and his infection was only picked up because of screening tests at Hong Kong airport. Genetic sequencing of the virus later showed he had been infected twice by different strains of Covid-19.

It makes sense that this is the case. If you think about the flu or common cold, there are various strains doing the rounds and you can often become sick with these multiple times in your life. Possibly even multiple times in a year.

It’s not always the case that the virus doesn’t cause symptoms the second time around, however. A 25-year-old man living in Nevada was infected with two variants of the virus SARS-CoV-2, which causes Covid-19, within a 48-day timeframe, according to details published in The Lancet Infectious Diseases.

In between both infections, he tested negative for the virus, according to his case report. When he caught Covid-19 again, his symptoms were more severe and he ended up in hospital needing oxygen support.

So, can we all get reinfected with Covid-19?

It’s complex, said Professor Daniel Altmann, an expert in immunology at Imperial College London. It’s not a matter of black and white, let’s say – there’s plenty of grey. “The problem is, this is a new virus we’ve had full knowledge of since January 11,” he told HuffPost UK in September.

“Also, we’ve never in our lifetimes had so many on the planet affected, so there’s an awful lot of diversity out there to understand.”

Since the start of the pandemic, Prof Altmann has spoken to a “steady trickle” of people convinced they’ve been infected by the virus twice. There’s a pattern he’s noticed: the person is unwell with classic Covid-19 symptoms and they have a test, but the test comes back negative. Then a month or two later, they become unwell again, with slightly different symptoms and a positive test.

“That sounds rather like reinfection to most of us,” he said. But it’s hard to distinguish between this hypothesis of reinfection, and the obvious alternative: that the first infection never went away and was just lingering to re-emerge.

“We always advised the only way to prove this was to compare the swabbed virus from the two attacks,” he says. “If they’re slightly different, that proves two separate infections. That has now been done in Hong Kong, formally, proving that it can occur.”

It’s good to put this information into context, though. There have been millions of cases of Covid-19 worldwide, and the Hong Kong case was the first reported case of reinfection.

How likely is it that we’ll get reinfected?

We can’t yet say how common reinfection is. “There is little data on which to base a guess,” said Prof Altmann, “but probably one in several thousand of those who have been infected.” So the likelihood of someone catching it twice is still pretty rare, in his eyes.

Immunity is complex, though, and depends on many factors, including how severely you had the virus the first time. Studies have shown those hospitalised with Covid-19 tend to have higher numbers of antibodies in their system than those with mild illness, potentially making them less likely to catch it again.

Based on what we know about antibodies, Professor Paul Hunter, from University of East Anglia, suggests it’s likely those who experience mild illness will have a shorter duration of immunity than those with severe illness.

But we don’t know exactly how long antibodies last – whether you have a mild or severe illness. Studies suggest they last in the system for at least four months, which means after that time, reinfection might be possible.

It’s likely reinfection would not cause as severe an illness the second time around, though, “because of some degree of residual immunity, which may not be sufficient to stop the infection, but enough to reduce the risk of severe illness,” said Hunter.

Antibodies aren’t the body’s sole defender, though – T-cells and B-cells, which the body releases as part of its immune response, could also play a part in fighting off another wave of illness. And these can remember the virus for much longer than a few months.

“We, and many others, have been looking in enormous detail at variations in types and levels of immunity in those who’ve been affected by Covid-19 to varying degrees,” said Prof Altmann. “The answer is: there’s enormous diversity out there, from those who have great levels of all the necessary types of immune cells, to those who have virtually no detectable immunity.”

What are the implications of this on a vaccine?

Prof Altmann doesn’t believe we need to worry. “The best vaccines are good at overcoming the foibles of poor immunity and susceptibility to natural infection, leaving the vast majority with a high enough level of protection,” he said.

Professor Brendan Wren, an expert in microbial pathogenesis at London School of Hygiene & Tropical Medicine, agreed. “It is to be expected that the virus will naturally mutate over time,” he said. “This [the Hong Kong case] is a very rare example of reinfection and it should not negate the global drive to develop Covid-19 vaccines.”