What are the risks of delaying the second COVID-19 vaccine dose?

Elianna Lev
·4 min read
A man receives his COVID-19 vaccine at a clinic at Olympic Stadium marking the beginning of mass vaccination in the Province of Quebec based on age in Montreal, on Monday, March 1, 2021. (Paul Chiasson/The Canadian Press via AP)
A man receives his COVID-19 vaccine at a clinic at Olympic Stadium marking the beginning of mass vaccination in the Province of Quebec based on age in Montreal, on Monday, March 1, 2021. (Paul Chiasson/The Canadian Press via AP)

As COVID-19 vaccines are being rolled out across Canada and around the world, the course of action for distribution is taking shape differently in different regions.

British Columbia announced that it will be increasing the time between the first and second doses of three of the COVID-19 vaccines, which prompted the country’s chief science adviser to label the move as a "population level experiment.”

Meanwhile, the U.S. government’s top infectious-disease expert Dr. Anthony S. Fauci echoed a similar sentiment, telling the Washington Post that switching from one to two doses to accommodate more people would be “a messaging challenge.”

However, some medical experts say that since vaccines are the most effective tool we have to control the situation in the middle of a global pandemic, there’s a trade off that has to be considered.

“Do we proceed with the two dose regimen, with the three or four week spacing for the specific vaccines, which will take longer to get to more people, or considering the current global supply issues with vaccines doses, do we move quickly to get as many people with one dose as possible,” says Vivek Goel, a professor at the Dalla Lana School of Public Health at the University of Toronto.

He says back in January, when the vaccines were beginning to be rolled out, there wasn’t a lot of data to inform decision making, so officials moved cautiously with the four week rollout.

However, in the last few months and weeks, there have been a number of reports, including data within Canada, of the level of efficacy a single dose has, which shows a considerable amount of protection at the individual level.

“It’s not as good as you’d get with two doses, but it's better than nothing,” says Goel. “Right now we’re probably better off exploring strategies that get to more people with the effectiveness that we get after a single dose than trying to pursue the two dose strategy with the two dose intervals that will slow us down. “

B.C. isn’t the only region delaying the rollout of the second dose. The U.K. is moving to the same approach, in an attempt to get maximum impact to the largest proportion of the population in the fastest period of time.

In terms of risk, Goel says there are risks all throughout the process. Since the vaccine isn’t 100 per cent effective, even with a second dose, there’s always the risk of being infected.

“For individuals, not getting a second dose right away does mean that there’s a lower level of protection, but they’re going to be advised to continue practicing protective measures regardless of if they get one dose or two doses,” he says.

Trying to avoid being infected should still be the priority.Dr. Reggie Lo

Dr. Reggie Lo, professor emeritus at the University of Guelph, concurs.

“Having one dose or two doses is not a free pass to return to normalcy,” he says. “The virus is not going away, and until we have the epidemic under control globally, we should continue with all the public health guidelines to minimize the chances of getting infected. The vaccine is protection in case you get infected, trying to avoid being infected should still be the priority.”

Another risk to consider is when you have partial immunity developed in between the doses, in some cases, the virus can mutate into a vaccine-resistant form. But Goal says the balance against that is that if we don’t get the diseases under control, we’re risking having more transmission of the variants in any case.

Lo says that there is no optimum time span to administer the two doses of the vaccine.

“When Moderna did the Phase 3 trial, it is likely that time was of the essence and they went with the four week span between shots,” he explains. “If they had more time, they might have tried a longer time span between shots to examine effectiveness. Hence the four week between shots could be an arbitrary time based on the trial conducted.”

He adds that there are other vaccines that have a longer time span between first and second shot, and over a range of months. But based on the human immune response, it is probably acceptable to have a time span of eight to 10 weeks between shots before losing immune protection.

Goal says he wouldn’t describe B.C.’s decision to delay the second dose as an experiment.

“You can’t do a trial on every single decision that has to be made in a situation like this,” he says. “It’s easy to say we should do a trial of six weeks versus three weeks or four months verses, but that’s going to take another six month to a year to do. How many people could die while we waited for that? These aren’t easy decisions for anyone to have to make but these are the things (public health authorities) have to weigh.”