How long should you wait for your third COVID-19 vaccine dose?

Amid the rapid rise of the Omicron variant in Canada, health officials are encouraging Canadians to get their third dose of a COVID-19 vaccine for better protection.

But provinces and territories are enforcing different timelines for eligibility, so when exactly should Canadians aim to get their third jab?

“After six months of your second dose, you should get your booster, particularly if you're…over the age of 50 and are considered at risk of serious disease,” Angela Crawley, a scientist at the Ottawa Hospital Research Institute, told CTVNews.ca on Tuesday in a phone interview.

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Crawley is referencing the latest guidelines on booster doses released by the National Advisory Committee on Immunization (NACI) on Dec. 3. The committee strongly recommends that several populations receive the third dose of the COVID-19 vaccine at least six months after receiving their second dose. According to NACI, third doses “may be offered” to Canadians 18 to 49 years of age as well, based on where they live and individual risks.

At the moment, only mRNA vaccines – Pfizer-BioNTech Comirnaty and Moderna Spikevax – have been approved for use as third doses. In both cases, Health Canada has authorized the use of these vaccines at least six months after an individual has completed their primary vaccine course.

Despite recommendations to wait at least six months, several Canadian provinces have outlined third dose eligibility requirements that don’t force residents to wait that long. In Alberta, for example, individuals aged 18 and up can book their booster shot after just five months of getting their second dose. In provinces like Saskatchewan and Ontario, the interval is even shorter, at just three months.

“I think it is confusing for Canadians, for sure,” Dr. Nazeem Muhajarine, an epidemiologist at the University of Saskatchewan, told CTVNews.ca in a phone interview on Tuesday. “It seems like a happenstance of residency...determining whether you can get a booster dose.”

Both Muhajarine and Dr. Peter Juni, head of the Ontario COVID-19 Science Advisory Table, are calling for provinces that currently require residents to wait at least six months to opt for shorter interval times, due to the emergence and high transmissibility of Omicron.

“Everything has changed with Omicron, there are new rules of the game,” Juni told CTVNews.ca on Tuesday in a phone interview. “What was still current six weeks ago is obsolete now.”

Juni pointed to recent data from the United Kingdom suggesting that protection against infection from COVID-19 decreases after roughly three months of receiving the second dose of the Pfizer vaccine. According to Juni, this data most likely holds for those who received a second dose of Moderna as well. At the end of November, the U.K. government announced that all adults would be eligible to receive their third dose of a COVID-19 vaccine after just three months of receiving their second dose.

Muhajarine pointed to another study from Israel, demonstrating the effectiveness of a third dose of the Pfizer vaccine at reducing the risk of disease-related hospitalization and death. Involved in the study were participants vaccinated after at least five months of receiving their second dose. Israel also recently announced that it would be shortening the amount of time between offering second and third doses of the COVID-19 vaccine to three months, down from five months.

“The data that we are seeing really points to a much shorter interval as the optimum interval to give a third dose,” he said. “I think six months might be the real outside limit of that.”

While residents across the country must contend with booster eligibility requirements based on their province or territory of residence, Muhajarine said he still encourages those who have received their second dose of the COVID-19 vaccine at least three to five months ago to get the booster dose as quickly as possible.

“We are contending with a variant that is really good at evading vaccine-induced antibody immunity,” he said. “When the third dose…is in place, our immune response gets back up and is able to continue to protect [against] severe disease.”

WANING PROTECTION FROM INFECTION AFTER TWO DOSES

With a roll out of second doses that took place across the country over the spring and summer months, Juni said that most Canadians are now more than three months after completing their primary vaccine series.

Looking at preliminary data recently compiled by the province’s Science Table, it appears as though two doses of current COVID-19 vaccines offer relatively good protection against hospitalization and ICU admission, Juni said. The province, however, has been seeing a rapid decline in protection against infection since the emergence of Omicron.

“People who have had two doses and are more than three months after their second dose really need to behave as if they weren't vaccinated when it comes to their risk of infection,” he said. “Vaccine protection against infection is basically just diminishing to next to zero now.”

While this data is based on what Ontario is experiencing, Juni said other provinces will likely see the same thing.

“To suggest that this won't be the case is like suggesting to the province that gravity doesn't apply; it will be the case,” he said.

In British Columbia, all adults in the province recently became eligible to receive their third dose of the COVID-19 vaccine at least six months after receiving their second dose. While Omicron cases logged in B.C. so far are not as high as those recorded in Ontario or Quebec, the protection offered by third doses is crucial to reducing the severity of COVID-19 infection, said Muhajarine.

“All over the country, transmission is high…every province is having Omicron-driven surges,” he said. “Booster doses are what will protect people who are infected from getting serious COVID-19 that will require hospitalization and ICU bed use.”

THIRD DOSES AND PUBLIC HEALTH MEASURES NEEDED TO COMBAT FIFTH WAVE

This is why third doses of the COVID-19 vaccine are so important, explained Juni, particularly for those who are most vulnerable, including individuals who are older in age or have underlying health conditions. Still, the epidemiologist said he acknowledges that more than just vaccines are needed to curb the fifth wave.

“The effectiveness [of current vaccines] is not as perfect as it was for Delta, but it's good,” he said. “But right now, it’s by far not enough. We need the combination of an aggressive booster roll out with public health measures.”

Aside from booking an appointment for a third jab, Juni advises people to stay put and decrease contact with others as much as possible. This includes avoiding crowded indoor spaces such as bars and restaurants. He also recommends wearing a medical mask topped with a two-layered cloth mask for a snug fit and extra protection.

Crawley also reinforces general COVID-19 public health measures such as practicing physical distancing and hand hygiene. She also points to other ways of maintaining overall well-being, such as getting a good night’s sleep, staying hydrated throughout the day and eating a balanced diet.

Aside from that, both Muhajarine and Juni said they hope to see provinces update their third dose eligibility requirements to reflect a three-month interval between second and third vaccine doses in order to offer extra protection to residents sooner.

“The challenge we have is that these infections are so pervasive, so prevalent in the population,” said Juni. “Policies need to change.”