There is still no consensus among specialists, although many believe that contracting the disease provides protection, at least for a while.
“Being immunised means that you have developed an immune response against a virus such that you can repulse it,” Eric Vivier, professor of immunology at the Marseille public hospital system, told AFP. “Our immune systems remember, which normally prevents you from being infected by the same virus later on,” he explained.
For some viral diseases, such as measles, recovery gives you immunity for the rest of your life. However, for viruses like SARS-CoV-2, it takes about three weeks to create enough antibodies, and even then, they can provide protection for just a few months, according to Vivier.
Immunity to the new coronavirus has been a controversial topic. “We do not have the answers to that – it’s an unknown,” said Michael Ryan, executive director of the World Health Organization’s Emergencies Programme, when asked about how long Covid-19 immunity would last after recovery.
“We would expect that to be a reasonable period of protection, but it is very difficult to say with a new virus – we can only extrapolate from other coronaviruses, and even that data is quite limited,” he added.
For SARS, which killed about 800 people in 2002 and 2003, recovered patients remained protected, on average, for “about three years,” said the director of the Genetics Institute at University College London, François Balloux. “One can certainly get reinfected, but after how much time? We’ll only know retroactively,” he pointed out.
A recently released Chinese study, not yet peer-reviewed, found that monkeys that recovered from SARS-CoV-2 were not re-infected when exposed to the virus again. “But it doesn’t really reveal anything,” said Frederic Tangy, a researcher at the Pasteur Institute, noting that the investigation was done in just a month.
In South Korea, several patients who had the disease, tested positive for the virus again after they recovered. However, there are some explanations for this, warned the scientists.
Although it is not impossible that these individuals have been infected a second time, there is little evidence to support it. Most likely, according to François Balloux, the virus has never completely disappeared, remaining – dormant and asymptomatic – as a “chronic infection”.
As the tests for infection have not yet been perfected, it is also possible that these patients, at some point, tested “false negative” when, in fact, they still had the pathogen in the body. “This suggests that people remain infected for a long time, several weeks,” added François Balloux.
Another study recently published looked at 175 recovered patients in Shanghai, China, revealing different concentrations of antibodies 10 to 15 days after the onset of symptoms.
“But whether that antibody response actually means immunity is a separate question,” said WHO Emergency Program technical leader, Maria Van Kerhove. “That’s something we really need to better understand – what does that antibody response look like in terms of immunity.”
For researcher Frederic Tangy, “it is possible that the antibodies that someone develops against the virus could actually increase the risk of the disease becoming worse.” The most serious symptoms, he explained, occur later, after the patient has created antibodies.
At the moment, it is also unclear which antibodies are the most potent to fight the disease and whether age makes a difference. Given all these uncertainties, experts have doubts about the “group immunity” strategy.
This approach was used in Great Britain and Finland, while in Germany some experts adopted the idea of an “immunity passport”, which would allow people to get back to work.
But “it is too premature at this point,” noted Saad Omer, professor of infectious diseases at the Yale School of Medicine. “We should be able to get clearer data very quickly – in a couple of months – when there will be reliable antibody tests with sensitivity and specificity,” he stressed.
The idea of passports or immunity certificates also raises ethical questions, say the researchers. “People who absolutely need to work – to feed their families, for example – could try to get infected,” said François Balloux.