It is still difficult to say exactly how many people were actually infected with COVID-19 since many of them did not develop symptoms and were not tested.
Researchers know that presymptomatic people (infected, but not yet showing symptoms) can transmit the virus. However, it is unclear whether asymptomatic people spread the virus.
A Chinese study recently published in the journal Nature (New window) questions the idea that a person infected with COVID-19 is subsequently immune.
According to the researchers, the level of antibodies to COVID-19 was much lower in asymptomatic people than in infected people who experienced symptoms of the disease.
Asymptomatic carriers longer
In addition, the level of antibodies in asymptomatic people decreased rapidly after infection. About 40% of patients had no detectable antibodies eight weeks after recovery. In infected people with symptoms, only 13% had antibodies after two months.
This suggests that asymptomatic individuals have developed a weaker immune response to the infection than those with symptoms.
It was also noted that asymptomatic people were positive for COVID-19 on average for up to 19 days after infection, while people with symptoms could still be positive for up to 14 days after infection.
Thus, asymptomatic people may shed the virus longer than people with symptoms.
The study also found that even with no visible symptoms, 70% of those infected had lung abnormalities – mainly spots called frosted glass infiltrate
, frequently seen in acute lung disease.
No antibodies, so no immunity?
Dr. Samir Gupta, a clinician scientist at St. Michael’s Hospital in Toronto, is not surprised that the level of antibodies decreases over time. It is a natural response of the body, since the production of antibodies is an energy-consuming activity for the body, he explains.
COVID-19 may act like other coronaviruses that cause simple colds, says Dr. Gupta; infected people would only be temporarily immunized.
He was however surprised to find that up to 40% of asymptomatic people had no more antibodies two months later. The clinician notes that the number of people observed in this study is small, which could have an effect on the results.
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There are other components of the immune system that could influence the length of immunity, such as immune memory cells, adds Dr. Gupta. These cells memorize a pathogen and start to release antibodies when they meet it again. However, they are difficult to detect, said Gupta.
Tania Watts, professor of immunology at the University of Toronto, worries that asymptomatic people take longer to get rid of the virus.
Until we have a vaccine, I think we should have very clear recommendations for everyone to wear masks.
Collective immunity is not the solution
Some countries, such as Sweden, have suggested controlling the spread of COVID-19 by allowing the majority of the population to be infected to generate collective immunity. When the population reaches a certain infection threshold (at least 60%), the virus can no longer spread exponentially.
But according to Watts, after seeing the findings of the study, it would be unwise to bet on collective immunity.
This means that we may have to wait for a vaccine that will induce a stronger and more lasting immune response than collective immunity, believes Watts. I think it puts even more pressure on vaccine development.
This study also questions the use of serological tests to identify asymptomatic people who have been infected and the use of immunity passports
– which would allow people already infected with COVID-19 to have more social interactions. It is still difficult to tell a person if they are safe or not.
Source: Radio-Canada | Science
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