The idea of decreased immunity has gained momentum in recent weeks, with some countries using it to justify rolling out third-dose COVID-19 vaccine boosters to their populations. But immunologists say the concept has been largely misunderstood.
While the antibodies – proteins created after infection or vaccination that help prevent future invasions of the pathogen – stabilize over time, experts say this is believed to happen.
And that doesn’t mean we aren’t protected against COVID-19.
Jennifer Gommerman, an immunologist at the University of Toronto, said the term “waning immunity” has given people a false understanding of how the immune system works.
“Waning has this connotation that something is wrong and there isn’t,” she said. “It’s completely normal for the immune system to trigger a response where a ton of antibodies are made and a lot of immune cells are developed. And for now, that sort of thing is taking over.
“But it has to twitch, or you wouldn’t have room for subsequent immune responses.”
Antibody levels rise in the “primary response” phase after vaccination or infection, “when your immune system is loaded and ready to attack,” said Steven Kerfoot, associate professor of immunology at Western University. .
They then decrease from this “emergency phase”, he added. But the memory of the pathogen and the body’s ability to respond to it remains.
Kerfoot said that B cells, which make antibodies, and T cells, which limit the virus’s ability to cause serious damage, continue to work together to prevent serious illness long after a vaccine is given. Although T cells cannot recognize the virus directly, they determine which cells are infected and kill them quickly.
Recent studies have suggested that the T cell response is still robust several months after a COVID-19 vaccination.
“You could get a minor infection… (but) all of those cells are still there, which is why we are still seeing very stable efficacy when it comes to preventing serious illness,” Kerfoot said.
A pre-printed study published this week by Public Health England suggested that the protection against hospitalization and death remains much higher than the protection against infection, even in the elderly.
So the concept of declining immunity depends on whether you measure protection against infection or against serious illness, Kerfoot said.
Ontario reported 43 cases of hospitalizations among those fully vaccinated on Friday, compared to 256 hospitalized unvaccinated infections. There were a total of 795 new cases in the province that day, 582 among those who were not fully vaccinated or whose vaccination status was unknown.
British Columbia, meanwhile, has seen 53 fully vaccinated COVID-19 patients hospitalized in the past two weeks, compared to 318 unvaccinated patients.
“You will hear people say that vaccines are not designed to protect infections, they are designed to prevent serious illness,” Kerfoot said. “I wouldn’t necessarily say it’s the vaccine that’s designed to do either… that’s exactly how the immune system works.”
Moderna this week released actual data suggesting that its vaccine was 96 percent effective in preventing hospitalization, even in the midst of the more transmissible Delta variant, and 87 percent effective in preventing infection – down by compared to the 94 percent efficacy seen in the latest clinical trials. year.
Moderna CEO Stéphane Bancel said the drop “illustrates the impact of declining immunity and supports the need for a booster to maintain high levels of protection.”
Pfizer-BioNTech backed the same with its own data, and a U.S.-based Food and Drug Administration advisory group voted on Friday to approve third doses for people aged 65 and over or above. risk of serious illness.
However, the panel rejected boosters for the general population, saying the drug company provided little safety data on the additional jabs.
Gommerman said the efficacy data presented by Moderna does not indicate the need for a third dose.
“The fact that it protects 87% against infection is amazing,” she said. “Most vaccines can’t achieve this.”
Bancel said Moderna’s research, which has yet to be peer-reviewed, suggests that a booster dose may also prolong the duration of the immune response by increasing levels of neutralizing antibodies.
But Dr. Sumon Chakrabarti, an infectious disease specialist in Mississauga, Ont., Said looking only at the antibody response is misleading and could be falsely used to justify an endless number of callbacks.
Israel, which has opened third doses to its citizens, recently talked about administering fourth doses in the near future.
“This idea of declining immunity is being exploited and it’s really worrying to see,” Chakrabarti said. “There’s this idea that antibodies mean immunity, and it’s true … but the background level of immunity, enduring T cells, hasn’t been emphasized enough.”
While some experts argue that boosters for the general population are premature, they agree that some people would benefit from a third jab.
The National Advisory Committee on Immunization has recommended boosters for those who are immunocompromised, who do not develop a robust immune response from a series of two doses.
Other experts have argued that long-term care residents, who were a priority when the rollout began last December, may also soon need a third dose. The English study suggests that immunity may decrease in older groups, but not much, if at all, in those under 65.
Chakrabarti said a decrease in protection among older populations may be due more to “overlapping factors” including their generally weaker immune systems and life situations for people in long-term care. .
“These are the people most at risk of hospitalization,” he said. “Could (the time that has passed after their doses) play a role? Yeah maybe. “
While we still don’t know the duration of the immune response to the COVID-19 vaccination, Gommerman said immune cells typically continue to live in the bone marrow and produce small amounts of antibodies for “decades.”
“And they can be quickly mobilized if they encounter a pathogen,” she said.
This report by The Canadian Press was first published on September 19, 2021.
Melissa Couto Zuber, The Canadian Press