Here are some key conclusions:
Antibody levels against COVID-19 were significantly lower in asymptomatic carriers than in those with symptoms during active infection.
Antibody levels also dropped much faster in people who never had symptoms, and 40% had no detectable antibody eight weeks after healing, compared to 13% of symptomatic patients .
People with asymptomatic infections tested positive for an average of five more days than people with symptomatic infections – 19 days versus 14 days – suggesting that they lost the virus longer.
The study also found that despite the absence of external symptoms, 70% had lung X-ray detectable abnormalities at some point in the infection – mainly spots called “frosted glass opacities”, which can indicate inflammation or other signs of illness.
No antibody can signify immunity, but not necessarily
Dr. Samir Gupta, a clinician scientist at St. Michael’s Hospital in Toronto and an assistant professor of medicine at the University of Toronto, noted in an interview with CBC News Network earlier this week that the study was very small.
Gupta, who was not involved in the study, added that it was not surprising that antibody levels dropped a few months after infection. He said it was normal because the body is energy-consuming to maintain the antibodies it does not need.
What was “a little surprising,” he said, was the fact that 40% of people with asymptomatic infections had no detectable antibodies.
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However, according to Gupta, people are immune to coronaviruses that cause common colds for only a few months, and this may also be the case for the coronavirus that causes COVID-19.
On the other hand, he said, “Antibodies are not the whole story. ”
There are other components of the immune system that play a role, such as memory cells. They remember a pathogen and start releasing antibodies when they meet it again, but they are difficult to detect, said Gupta.
What it means for collective immunity and vaccines
However, Tania Watts, a professor of immunology at the University of Toronto who was not involved in the study, expressed concern about the implications.
“This suggests that natural infection may not confer lasting immunity, which worries people,” she said.
Some countries like Sweden and at least one Canadian province have already suggested that one way to control the spread of COVID-19 is to allow most of the population to be infected in a controlled manner to generate “Collective immunity”. Once the population reaches a certain threshold of previous infection, there will not be enough people left to spread the virus and it cannot spread exponentially as an epidemic.
But Watts said the low and short-lived antibody levels in asymptomatic infections in this study suggest that we cannot rely on herd-induced immunity long enough to have an impact.
This means that we may have to wait for a vaccine that will induce a longer and stronger response than many natural infections, she said. “I think it puts even more pressure on the development of vaccines. “
What it means for antibody testing, “immunity passports”
Watts said that another implication of the study is that serological (blood) or antibody tests – which have been touted as a way to get an idea of who has been previously infected, of the proportion of the population which represents and proximity to herd immunity. – may not work as expected.
And it throws cold water on the controversial idea ” immunity passports“, The idea of allowing more social interactions, such as work, travel and mass gatherings, for people who have already been infected and who are therefore immune and cannot spread the virus – which would be based on serological tests.
“Until we know which part of the immune system is protective,” said Watts, “it is difficult to be able to take a test and tell someone that you are safe or not.” “
What it means for disease transmission
Although it is known that presymptomatic people can transmit COVID-19, it is not clear if people who remain asymptomatic during the disease can.
Watts said she thinks the study’s conclusion that people without symptoms lose the virus longer than people with symptoms is “shocking” and suggests that we should be concerned about the transmission of asymptomatic people.
“Until we have a vaccine, I think we should have very clear recommendations for everyone to wear masks. ”
She said the longer period of viral shedding is likely due to the fact that a lack of symptoms indicates a weaker immune response, which results in a longer delay in clearing the infection.
On the other hand, an overly intense immune response is what makes intensive care patients struggle to breathe.
The ideal is somewhere in between and what we would like in a vaccine, said Watts.
“We really need this Gold Loop immune response. “