How effective is a single dose of a COVID-19 vaccine?

More and more Manitobans are getting inoculated against COVID-19, but the majority have just had a single dose so far.

As of Friday, 259,847 doses of vaccine have been administered in the province, including 192,131 first doses and 67,716 second doses.

But just how safe are you on a single dose of the vaccine?

A letter by two Canadian experts published in the New England Journal of Medicine (NEJM) earlier this year stated that with a 92.6 per cent efficacy, the first dose of the Pfizer vaccine was “highly protective.”

Several provinces, including Manitoba, have delayed their second doses in order to prioritize getting as many Manitobans vaccinated as possible.

Dr. Joss Reimer, the medical lead of Manitoba’s vaccine implementation task force, said they based that decision on studies in British Columbia, Manitoba, Quebec, the U.K. and Israel.

“All of them showed that the first doses in real-world effectiveness provided either in the 70s or 80s per cent protection,” Reimer said during a press conference Friday.

“Which in real-world numbers is highly effective. So we were very happy to see that after one dose, we would be very happy to see effectiveness in the 80s after two doses.”

Reimer also says the length of immunity based on a single dose was another reassuring factor.

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“The immunity lasted for the entire length of the studies. The studies were not very long because we hadn’t had the vaccine for very long at that point, but even the longest ones being two months there was no decrease in the immune response at that two-month mark,” she said.

“We felt confident that we had a clear minimum of two months protection, a very strong likelihood a three-month protection, but we truly think it will probably last more like six months if we look at data from other vaccines and how long we see responses to them.”

However, Reimer says some Manitobans have got the virus after receiving the vaccine.

“We have had individuals who have been infected with COVID after immunization,” Reimer said.

Epidemiologist Cynthia Carr says the vaccine trials were based on preventing severe outcomes of COVID-19, rather than stopping transmission.

“The clinical trials did not address how effective the vaccine was in stopping the transmission of the disease in preventing asymptomatic or mild illness,” Carr told Global News. “It was mostly based on very symptomatic, obvious illness, and the most severe outcomes.”

“So it would not be surprising, unfortunately, if someone who had their first dose could still become infected with COVID,” she added. “But typically what happens, even if the vaccine is not 100 per cent effective against preventing symptomatic disease, it typically has efficacy in preventing the most severe outcomes. So you had that first dose, your immune system has learned something and it can react in a better way typically.”

Carr also says the vaccine affects people differently.

“One size doesn’t fit all,” she said.

“We all have different immune systems and different risk and vulnerability. So again, a vaccine is not a treatment, a vaccine is a coach. It’s teaching your body, teaching your immune system what to do. So if your immune system isn’t good at communicating between the cells and reacting quickly and creating as much fighters as you need, you will have less protection.”

Public health officials are going to keep a close eye on what’s rolling out in other jurisdictions.

“If we see signs in any other jurisdiction that that immunity is going away, then we can start to move towards second doses for vulnerable Manitobans,” Reimer said.

“For today most jurisdictions are still seeing good protection after one dose. The best way for us to prevent this third wave or at least prevent it from being larger than it needs to be is to have as many people immune as possible. So even for those folks who have to wait for their second dose, they will be better protected if all the people around them have received a first dose.”