Why your 1st COVID-19 shot is more protective than you might think


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This is an excerpt from Second Opinion, a weekly roundup of health and medical science news emailed to subscribers every Saturday morning. If you haven’t subscribed yet, you can do that by clicking here.


After months of anxiously waiting, many newly vaccinated Canadians are left with more questions than answers — how protected am I after one dose of COVID-19 vaccine, when will I get my second shot and what can I do safely until then?

“It’s the elephant in the room,” said Dr. Zain Chagla, an infectious diseases physician at St. Joseph’s Healthcare Hamilton and an associate professor at McMaster University. “Everyone is asking about this.” 

While we still don’t have all the answers, there are some promising early signs that may help put your mind at ease until you get your second shot.

How protected am I until my second shot? 

A recent study in The Lancet looked at more than 23,000 vaccinated healthcare workers in the United Kingdom from December to February and found the Pfizer-BioNTech vaccine was at least 70 per cent effective at preventing COVID-19 three weeks after the first dose. 

Another Lancet study looked at more than 1.3 million people in Scotland during the same time period and found the Pfizer shot was more than 90 per cent effective at preventing hospitalization due to COVID-19 four to five weeks after the initial dose. 

People living in neighbourhoods deemed to be at a higher risk of COVID-19 transmission line up in the rain outside Downsview Arena in Toronto on Wednesday. (Evan Mitsui/CBC)

That study also analyzed the AstraZeneca-Oxford vaccine in the same population and found it was 88 per cent effective at curbing hospital admissions from COVID-19.

“People are thinking about vaccination right now that you have a cup that you’re filling half full with the first dose, and you’re getting it full with the second — but that’s not how immunity works,” said Dr. Sumon Chakrabarti, an infectious disease specialist at Trillium Health Partners in Mississauga, Ont.

“At four weeks and beyond you’re probably still mounting immunity and the second dose … takes your existing immunity and it kind of fortifies it.”

Alyson Kelvin, an assistant professor at Dalhousie University and virologist at the Canadian Center for Vaccinology and the Vaccine and Infectious Disease Organization in Saskatoon, likens that immune boost from the second shot to “restudying” for a test you might have coming up.

“It’s cementing the information into your memory and that’s what we need the boost for — to make sure that we have that long lasting response and antibodies that will protect us,” she said. “But we need to go back for the boost to make sure that that is retained for longer periods of time.”

While we still don’t know how long that initial immune response lasts, Kelvin said we can extrapolate from previous vaccine studies that the second dose is generally more effective the longer you wait between shots. 

“This is not the first vaccine that we obviously have that’s multiple doses,” said Chakrabarti. “With the Hepatitis A vaccine you can often give the second dose anywhere from six to 36 months later.” 

When will I get my second shot? 

In March, Canada’s National Advisory Committee on Immunization (NACI) recommended delaying second doses of COVID-19 vaccines by up to four months — the longest known interval by any country in the world so far. 

NACI said it based its revised guidelines on emerging real world evidence and the reality of Canada’s limited supply of vaccines at the time, as well as findings from the British Columbia Centre for Disease Control that determined one dose of the vaccine was actually more effective than clinical trials had initially shown. 

“There are profound individual effects with that first dose. They’re not as good as a two-dose strategy and we all recognize that,” said Chagla. “But I think when NACI moved to this strategy, the data wasn’t really there on what that looks like in the long term.” 

WATCH | The science behind delaying the 2nd dose of COVID-19 vaccines:

Federal government scientists have put their support behind delayed second doses of COVID-19 vaccines — which several provinces were already doing — and ongoing research shows some of the benefits of the adapted strategy. 2:04

Now we’re learning more about how effective even that first dose is, Chagla said, and we’re also getting a glimpse of whether COVID-19 vaccines can prevent transmission onto other people. 

A new preprint study by Public Health England, which has not yet been peer-reviewed, found that three weeks after a single dose of either the Pfizer or AstraZeneca vaccine, household transmission of the virus was reduced between 39 and 49 per cent.

Chakrabarti said this is a “great sign” that is part of a growing body of research that suggests even just the first dose of a COVID-19 vaccine significantly reduces the spread of the virus and the severity of disease in those who do get infected. 

“This goes to show why initially that first dose for as many people as possible was the better strategy to go by because it still is going to give good protection,” he said. 

Can I get COVID-19 after my first dose? 

Canada’s Deputy Chief Public Health Officer Dr. Howard Njoo said during a press conference Thursday that as more Canadians get vaccinated, there are bound to be some cases of infections after the two week window it takes to build antibodies.  

“No vaccine is 100 per cent effective,” he said. “At a high population level with millions and millions of Canadians being vaccinated, certainly we do anticipate that there will be cases occurring.” 

A new study from Imperial College London published in the journal Science Friday found that a single dose of the Pfizer vaccine may not generate an adequate immune response to protect against variants of concern, except for people who already had COVID-19.

But that study was based on laboratory testing of blood samples from healthcare workers in the U.K. and it may not necessarily translate to the real world. 

Nevertheless, Njoo said Canadian public health officials at all levels are studying the issue of “vaccine escape” from the variants closely to determine whether they could jeopardize vaccine effectiveness, but more research is needed.

Deputy Chief Public Health Officer Howard Njoo says as more Canadians get vaccinated, there are bound to be some cases of COVID-19 infections after the two week window it takes to build antibodies. (The Canadian Press)

What can I safely do with just one dose? 

Close to one-third of the Canadian population has now received at least one dose of a COVID-19 vaccine, but Canada still hasn’t provided any formal guidance on what they can or can’t do while waiting for their second shot.

Many Canadians are left wondering if they can safely see their families, other vaccinated people or generally feel less at risk from COVID-19 after a year under strict public health measures.

Chagla said that giving people realistic public health guidance on what they can do safely with their first dose of vaccine could go a long way to prevent them from making up their own rules in the interim.

“Use it as part of a bundle — have potentially slightly higher outdoor gathering limits amongst people who are vaccinated as long as they adhere to public health precautions,” he said.

“Because the bundle of being vaccinated plus distancing, plus the outdoor ventilation kind of all lines up to reduce the risk for that individual.”

The U.S. Centers for Disease Control and Prevention updated guidelines this week for fully vaccinated individuals, saying they don’t have to wear a mask outdoors in certain settings or indoors with others who have received both shots.

WATCH | Provinces expand vaccine eligibility as supply set to spike:

As Canada prepares for a huge increase in COVID-19 vaccine deliveries, more provinces are announcing plans to expand vaccine eligibility, including Quebec, Ontario and B.C. 1:56

But unlike Canada, the U.S. hasn’t delayed second doses by up to four months. Answers to those questions have been harder to come by for Canadians months after guidelines changed and close to 15 million doses have been administered.

“We’re in the middle of the third wave and we have a wall of COVID in front of our faces but I think it’s very important that we need to have a roadmap and a framework out of here,” Chakrabarti said.

“We know people are getting together behind closed doors and I think that what would be a better approach than to just constantly be telling people to stay home … you have to have a level of trust for the Canadian public at this point.”


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