Can people who have recovered from the coronavirus disease (Covid-19) get reinfected? There is no evidence of reinfection, say experts, but since the disease is just a little over six months old, it will take years to get data on how long a recovered patient will remain protected against reinfection.

Monkeys infected with the coronavirus were protected from reinfection for up to 28 days, according to a study from China published in the journal Science on Friday. Six rhesus macaques were infected with Sars-CoV-2, the virus that causes Covid-19, in their windpipe (trachea) developed mild to moderate symptoms and recovered within two weeks. When four of the six monkeys were reinfected 28 days after the first infection, the monkeys showed a stronger immune response , producing more neutralising antibodies which protected them against short-term reinfection.

“There is experimental evidence from monkeys that once infected, there is resistance to reinfection. In humans, reinfection has not been observed in recovered patients, despite many months since the first wave of infections. We do not know that this is necessarily attributable to antibodies, but rise of antibodies is correlated to disappearance of the virus. So, in a nutshell, there is a reason to believe that recovery from infection is associated with gaining at least short-term immunity; antibodies contribute to such immunity, and antibodies are not the full story,” said Dr Anurag Agrawal, director, Institute of Genomics and Integrative Biology (Council of Scientific and Industrial Research, Delhi.

Studies have found that levels of neutralising antibodies that bind to the new coronavirus viral proteins to directly prevent infection remain high only for a few weeks after infection, after which it begins to fall. Antibodies last at higher levels for longer in people who had severe disease, compared to those with mild or asymptomatic disease, which has led to concerns that those with mild disease may not have lasting protecting against Covid-19.

This follows a pattern similar to other coronavirus diseases, such Sars (severe acute respiratory syndrome). Most recovered patients lose their neutralising antibodies within a few years, but those with severe disease had antibodies 12 years after infection.

What is known about Sars-CoV-2 infection is that no reinfection has been reported since the pandemic started in China in December 2019. “As of now, no evidence of reinfection has been validated, and theories on when antibodies in people with mild and asymptomatic disease disappear are pure guesswork. The few reported cases of reinfection from Korea and some other countries were misrepresentation. They found traces of viral genetic material (RNA) in nasal swabs, which may have been a remnant of natural infection before recovery. RNA doesn’t cause disease or spread infection. When the viral load is low and there is no transmission, it is of no epidemiological significance as the person doesn’t get sick or infect others,” said Dr T Jacob Jon, virologist and professor at Christian Medical College, Vellore, Tamil Nadu.

The levels of neutralising antibodies needed to prevent reinfection or reduce Covid-19 symptoms in a second illness is also unclear. “We do not know how long protection lasts and measuring antibodies does not necessarily fully cover the immunity. Cell-mediated immunity seems important too. Most mutating viruses need repeat vaccination. This is mutating slower than expected but yes, it seems unlikely that there will be a one-time only vaccine,” said Dr Agrawal.

A comparison of the immune responses of 37 asymptomatic patients to an equal number of severely ill patients in China showed asymptomatic people reacted less strongly to infection, 40% of asymptomatic patients losing their IgG protective antibodies—the slower-forming antibodies associated with long-term immunity—within a few weeks or months after recovery, within two to three months after the infection compared to 12.9% of the symptomatic patients, reported a study published in Nature Medicine on June 18.

It’s still unknown if the lack of antibodies months or years later will put people at risk of reinfection, but testing for them helps understand the spread of infection. “Sero-surveillance definitely helps predictions and policy. For the person tested, given that reinfection is not seen, it is likely that those who have antibodies are already recovered and have lower risk. However, this is not proven and antibody positivity does not equal immunity in many diseases. This may be resolved by testing for neutralising antibodies but it is equally possible that even those without neutralising antibodies may have antibody-independent immune resistance acquired during prior infection,” said Dr Agrawal.

The bottom line? “If I were tested, I would be hoping for a positive test,” said Dr Agrawal.