Scientists at the National Advisory Committee on Immunization are reviewing research that suggests people who have been infected with COVID-19 can turbocharge their antibodies with just one dose of a vaccine.
The committee is "actively reviewing evidence on the protection offered by one dose for those previously infected, and whether a second dose continues to be necessary," says a statement from the panel.
Caroline Quach-Thanh said the committee is "debating" the question of how many vaccine doses someone who has been infected with COVID-19 requires.
"France and Quebec have said only one," said Quach-Thanh, chair of the committee, in an email.
Quebec's Health Department confirmed Thursday that previously infected patients will only require a single dose of vaccine, a decision made by experts at the province's public health institute and the province's immunization committee.
They said those with a confirmed infection have built up an immune memory during infection, so a single dose of vaccine acts as the booster.
Authorities added that Quebecers previously infected will be able to secure a second dose if they want one, and anyone immunosuppressed who has been infected should get both doses.
Studies have suggested people who have had COVID-19 may produce the required antibody response with just one dose of a vaccine.
A letter in the New England Journal of Medicine earlier this month says the question of whether one dose is enough "requires investigation." It's written by 32 researchers from the Icahn School of Medicine at Mount Sinai in New York after conducting a small study.
It suggests people who have been infected with the novel coronavirus may produce 10 to 45 times as many antibodies after the first dose of the Pfizer-BioNTech or Moderna vaccines compared with someone who hasn’t had COVID-19. The research is yet to be peer reviewed.
Quach-Thanh said data shows that the response to the first dose of vaccine is strong for those who have been infected with the virus.
"Like a booster dose," she said.
Those who have been infected and get a second shot may also have stronger side-effects, such as fever, aches and feeling tired, which are signs their immune system is already primed, Quach-Thanh said.
"The question that remains is: is that true for everyone or at least for the vast majority?"
Dr. Bonnie Henry, British Columbia’s provincial health officer, said early data from around the world suggest people who have had a lab-confirmed positive test for COVID-19 get a good response from just one dose of a vaccine.
“The jury is still out but more and more it is looking like they get a really strong booster effect from a single dose and a second booster may not be necessary,” she said.
Quebec’s public health director, Horacio Arruda, told a news conference earlier this week that immunization experts believe a single dose of vaccine, when given three months after recovery from the disease, provides the same amount of immunity as two doses.
A second dose, Arruda explained, is not recommended for people who have had COVID-19 because "it doesn't give more immunity, and it brings more significant adverse effects," such as flu-like symptoms.
Prof. Fiona Brinkman of Simon Fraser University's molecular biology and biochemistry department said the number of antibodies produced by a person may depend on how severe their COVID-19 infection was.
That does not mean people need to get an antibody test done before getting a vaccine, although that may change, she added.
"Right now, the policy is that we're just vaccinating everybody and doing that first dose."
However, Quach-Thanh warned that the presence of antibodies is not, in itself, a sign of protection and they are all not created equal.
"Some people without much antibodies will have protection, while others with antibodies may not be that well protected," she said.
Brinkman said it's also possible that there may be a difference in the kind of antibodies produced after a COVID-19 infection and a vaccine.
"The most important thing is to have these certain antibodies — what we call neutralizing antibodies — that we really want," she said, adding the vaccines have been shown to produce them.
The experts said there is no harm in those who have been infected with COVID-19 getting both shots.
The statement from the committee said it is also evaluating how long someone previously infected with COVID-19 could wait before getting a vaccine, based on emerging evidence.
Brinkman said one of the concerns researchers have is how long immunity might last.
"This disease literally hasn't been around long enough to allow us to appropriately assess how long you have antibodies and an immune response that will be effective against this virus, if you've either been vaccinated or you've had the disease."
Researchers said it could also potentially mean that the vaccine can be delivered into arms faster because a number of people may only need one dose of the shot.
"It really is good news," Brinkman said. "If anything, it's a good thing."
