Booster shots, 3rd doses and who needs them

October 29, 2021
Booster shots, 3rd doses and who needs them
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As we approach the 11-month mark from when the first COVID-19 vaccinations were given in Canada, many people are wondering about whether — and when — they might need an additional dose to remain protected against the virus. That raises lots of questions. Here’s what we know — and what we don’t.  

What’s the difference between a booster shot and a third dose?

The three approved COVID-19 vaccines in Canada (Pfizer-BioNTech, Moderna and AstraZeneca) are all two-dose regimens. A third dose is for people who may not have mounted a strong enough immune response to fight off COVID-19 after two doses. 

The National Advisory Committee on Immunization (NACI) has identified two populations that require third doses: elderly people living in long-term care and those who have specific health conditions that make them immunocompromised (including organ transplant recipients). For these people, a third dose is an extension of their primary series of vaccinations. 

A booster shot is for people who likely had a fulsome immune response to the regular two-dose vaccine regimen, but “with time, the immunity and clinical protection has fallen below a rate deemed sufficient in that population,” according to the World Health Organization (WHO).

It’s comparable to the tetanus vaccine, which requires a booster shot every 10 years. Most experts agree that everyone will likely need a COVID-19 booster shot at some point within the next year, but the big question is when.

How do you know if it’s time for a booster?

Researchers, immunologists, infectious disease specialists, public health units and all levels of government are constantly looking for signals that would suggest immunity is waning in certain parts of the population. One potential signal is an increase in breakthrough infections among people who have been fully vaccinated. That happened in some long-term care homes and was part of the reason why experts decided residents should get a third dose of the vaccine. 

Booster shots should be given only to targeted populations where the science shows they’re likely needed, WHO said in a statement in early October.  

“The rationale for implementing booster doses should be guided by evidence on waning vaccine effectiveness, in particular a decline in protection against severe disease in the general population and in high-risk populations, or due to a circulating VoC [variant of concern],” WHO’s statement said. 

Right now, many experts say, most Canadians still have very good protection against COVID-19, including the delta variant. 

“We’re not seeing any risk that people who are vaccinated, outside of those special populations [identified by NACI] are having any waning immunity,” said Dawn Bowdish, Canada Research Chair in aging and immunity and a professor at McMaster University in Hamilton. 

Why are some provinces and territories giving boosters to more people than others?

On Thursday, the Northwest Territories announced anyone 18 years of age or older will now be eligible for a booster shot.  

Last week, British Columbia said that seniors age 70 and older, all Indigenous people 12 and older and health-care workers who only had three or four weeks between their two doses will all be able to get a booster shot by the end of the year. By next May, everyone in B.C. will be eligible for one. 

WATCH | B.C. to offer COVID-19 booster shots to everyone by May:

B.C. to offer COVID-19 booster shots to everyone by May

B.C. is the first province to announce a plan for COVID-19 booster shots, with all residents being eligible in May, or six to eight months after their first dose. 1:53

In Alberta, Indigenous people aged 65 and older can get a booster shot and other seniors can get one if they’re 75 and older. (Indigenous people are at disproportionate risk of getting seriously ill with COVID-19 due to several factors, including socio-economic marginalization and barriers to accessing care, according to the United Nations.)   

Ontario’s guidance is similar to NACI’s and the province is providing boosters to people in long-term care and those who are immunocompromised.   

The reason for the wide variability across the country is that most aspects of health care, including vaccination, are under provincial or territorial jurisdiction. NACI makes recommendations on how vaccines should be administered, but provinces and territories ultimately make their own decisions. 

Why doesn’t Canada just offer boosters to everyone?

Health officials are trying to walk the line between giving third doses and boosters too early and giving them after people have already fallen ill, said Dr. Allison McGeer, an infectious diseases specialist and microbiologist at Sinai Health Systems in Toronto.   

Because it’s not yet known how long an effective immune response from the first two doses will last, health officials are trying to figure out the optimal time for a booster, so people get the longest-lasting protection possible out of each dose.  

Another big reason, many health experts say, is “vaccine equity.” Much of the world, particularly Africa, hasn’t even received a first dose yet, according to WHO.  

WHO has repeatedly pleaded with rich countries to stop giving what it considers unnecessary booster shots to people who are already protected against COVID-19, so those vaccines can be redirected to developing countries. 

“We’re planning to hand out extra life-jackets to people who already have life-jackets, while we’re leaving other people to drown without a single life-jacket,” Dr. Mike Ryan, head of the World Health Organization‘s health emergencies program, said during a news conference in August. 

WATCH | WHO calls for halt on COVID-19 booster shots: 

WHO calls for temporary halt on COVID-19 vaccine booster shots

Saying the priority should be on immunizing health-care workers in low- and middle-income countries, WHO director general Tedros Adhanom Ghebreyesus called for a temporary moratorium on COVID-19 vaccine boosters. 1:44

Many infectious disease and immunology experts agree that in addition to the ethical considerations, it’s in Canada’s best interest to help ensure people in developing countries get the COVID-19 vaccines they need. 

That’s because the longer the coronavirus that causes COVID-19 can circulate among unvaccinated people, the more likely it is to mutate into new variants that put everyone at increased risk. 

“These vaccines are precious,” Bowdish said. “If we want to make sure there’s not the next variant and the variant after that and the variant after that, we really need to get the world vaccinated.”

I keep hearing about ‘waning immunity.’ Does that mean I need a booster?

The term “waning immunity” usually refers to a decrease in antibodies over time. After any vaccination, immunologists and virologists say, it’s normal for antibody levels to go down.

But the immune system is much more than antibodies. B cells and T cells are also hard at work. B cells produce antibodies and if they’re exposed to the virus again, they “remember” and can make more. T cells target and get rid of infected cells.

A study published in the journal Science found “robust cellular immune memory” from B cells for at least six months after mRNA vaccination against all circulating strains of the virus — even the highly contagious delta variant.

Many experts agree that having waning antibodies does not automatically translate into a need for a booster shot. 

Who should be next in line for booster shots?

Some infectious disease specialists and immunologists have suggested that front-line health workers who received their vaccinations several months ago should be on NACI’s list. 

Like long-term care residents, most health-care workers were vaccinated in December 2020 or near the beginning of 2021, meaning up to 10 months have passed. In addition to the possibility that their immunity has waned over time, they also had their second shots three or four weeks after their first, because that’s what the vaccine manufacturers (Pfizer-BioNTech and Moderna) recommended. 

But there’s now strong evidence from research studies out of B.C. and Quebec suggesting that the Canadian government’s decision to expand the length of time between doses resulted in a more robust immune response. The health-care workers who were vaccinated before that decision was made don’t have this extra protective factor.

Other experts have suggested that elderly people who don’t live in long-term care should be the next population to get boosters.  

Dr. Samir Sinha, director of geriatrics at Mount Sinai and University Health Network hospitals in Toronto, is especially concerned about “frail, older, home-bound individuals who have lots of people providing their care.”

Different care providers coming in and out of their homes could pose a similar risk of COVID infection that people in long-term care homes face, Sinha said.



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