A closer look at people who tested positive for COVID-19 but never developed symptoms has found that such asymptomatic carriers have few to no detectable antibodies just weeks after infection, suggesting they may not develop lasting immunity.
There’s growing evidence that a significant proportion of people who test positive for COVID-19 never show symptoms, although it’s not clear what percentage of people that is and what role they play in spreading the disease.
A Chinese study published this week in Nature followed 37 people in Wanzhou District in China who did not show any outward signs of the disease, despite testing positive when their respiratory tracts were swabbed and being kept in hospital for observation.
Some key findings include:
- Levels of antibodies against COVID-19 were significantly lower in asymptomatic carriers than those with symptoms during active infection.
- Antibody levels also dropped off far more quickly in people who never showed symptoms, and 40 per cent of them had no detectable antibodies eight weeks after recovery, compared with 13 per cent of symptomatic patients.
- Those with asymptomatic infections tested positive for an average of five days longer than people with symptomatic infections — 19 days compared with 14 days — suggesting that they were shedding the virus longer.
The study also found that despite having no outward symptoms, 70 per cent had lung abnormalities detectable in X-rays at some point during infection — mostly spots called “ground-glass opacities,” which can indicate inflammation or other signs of disease.
No antibodies could mean no immunity, but not necessarily
Dr. Samir Gupta, a clinician-scientist at St. Michael’s Hospital in Toronto and 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 wasn’t involved in the study, added that it wasn’t surprising that antibody levels fell a few months after infection. He said that’s normal, since it’s energy intensive for the body to maintain antibodies it doesn’t need.
What was “a little bit surprising,” he said, was the fact that 40 per cent of people with asymptomatic infections had no detectable antibodies at all.
However, Gupta said, people have immunity to coronaviruses that cause common colds for only a few months, and that may also be the case for the coronavirus that causes COVID-19.
On the other hand, he said, “antibodies aren’t the whole story.”
There are other components of the immune system that play a role, such as memory cells. They remember a pathogen and begin releasing antibodies when they encounter it again, but they are hard to detect, Gupta said.
What this means for herd immunity and vaccines
Still, 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 give long-lasting immunity, which is what people have been worried about,” she said.
Some countries such as Sweden and at least one Canadian province have previously suggested that one way to control the spread of COVID-19 is to allow most of the population to get infected in a controlled fashion to generate “herd immunity.” Once the population reaches a certain threshold of previous infection, there won’t be enough susceptible people to spread the virus, and it can’t spread exponentially as an epidemic.
But Watts said the low and short-lived levels of antibodies in asymptomatic infections in this study suggest we can’t rely on herd immunity being induced for long enough a period of time to have an impact.
That means we may need to wait for a vaccine that induces a stronger, longer-lived response than many natural infections, she said. “I think this puts even more pressure on vaccine development.”
What this means for antibody tests, ‘immunity passports’
Watts said 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, how much of the population that represents and how close that is to herd immunity — may not work as hoped.
And it throws cold water on the idea of controversial “immunity passports,” the idea of allowing more social interactions, such as work, travel and mass gatherings, for people who have previously been infected and therefore are immune and can’t spread the virus — which would be based on serological testing.
“Until we know what part of the immune system is protective,” Watts said, “it’s difficult to be able to do a test and tell someone you’re safe or not.”
What this means for disease transmission
While it’s known that presymptomatic people can transmit COVID-19, it’s not really known whether people who remain asymptomatic through the course of the disease can.
Watts said she thinks the finding in this study that people without symptoms shed the virus longer than people with symptoms is “shocking” and suggests we need to worry about transmission from asymptomatic people.
“Until we have a vaccine, I think we should have very clear recommendations that everybody wears masks.”
She said the longer period of viral shedding is probably because a lack of symptoms indicate a weaker immune response, resulting in a longer time to clear the infection.
On the other hand, too intense an immune response is what puts patients in the ICU struggling to breathe.
The ideal is somewhere in between and what we’d like in a vaccine, Watts said.
“We really need that Goldilocks immune response.”