What do we know about COVID-19 vaccine booster shots?

A mosque in Cairo, Egypt. (Dr. Noorali Bharwani)
A mosque in Cairo, Egypt. (Dr. Noorali Bharwani)

Like many of you, I am curious to know what kind of protection am I getting after third and fourth dose of COVID-19 vaccine.

Is it necessary to have third and fourth dose?

What is meant by “fully vaccinated” against COVID-19?

How many vaccine doses do we need to remain protected against COVID-19?

As you may know, some medical conditions, such as measles or polio, completing the primary vaccination series usually provides lifelong protection against disease. For others, such as tetanus, diphtheria, and pertussis, periodic booster doses of vaccine are required.

With COVID-19 we have learnt levels of protection begin to wane over time, resulting in breakthrough infections. Breakthrough infection is usually not serious.

Many experts believe people are “fully vaccinated” either two weeks after they receive their second dose in a two-dose series, or two weeks after their first dose for single-dose vaccines.

However, “fully vaccinated” is not the same as “optimally protected”, says CDC (US Center for Disease Control and Prevention). “To be optimally protected, a person needs to get a booster shot when and if eligible.”

We know one- or two-dose series remains extremely effective at preventing severe infection and death. How can we maintain this level of protection?

On 30 March, 2022 the CDC recommended a fourth dose of COVID-19 vaccine for everyone over 50 years of age.

No vaccine is 100 per cent effective. Breakthrough infections can happen with every vaccine, and do not mean that the vaccine does not work.

According to data from the US CDC, unvaccinated people are at 11 times the risk of death from COVID-19 than vaccinated people.

WHO (World Health Organisation) says, “Even once you are fully vaccinated, continue to practice the same prevention measures to protect yourself. Stay at least one metre away from other people, wear a well fitted mask over your nose and mouth when you can’t keep this distance, avoid poorly ventilated places and settings, clean your hands frequently, stay home if unwell and get tested.”

Why do we need booster dose?

The protection you get from COVID-19 vaccines can wane over time (4-6 months); so, booster doses are necessary. Boosters can help improve protection against severe outcomes by up to 90 per cent. They may also reduce the risk of post COVID-19 condition.

Many Canadians are reluctant to go for booster shots (third and fourth shot). According to an article in the Canadian Medical Association Journal (CMAJ April 11, 2022) only 56 per cent of adults, and far fewer children, have received three doses of a SARS-COV-2 vaccine. Meanwhile, vaccine manufacturers are warning that fourth doses may be necessary to maintain immunity.

Third dose is 95 per cent effective against hospitalization and death. It also brings vaccine efficacy to 97 per cent for Delta and 61 per cent for Omicron. However, it’s unclear how long good protection from a third shot will last.

Is fourth vaccine dose necessary?

Canada’s National Advisory Committee on Immunization currently recommends four doses for people who are immunocompromised, but most provinces are also offering fourth shots to seniors. Both groups may face increased risks of severe illness and greater declines in vaccine efficacy than the general population. Otherwise, experts remain divided on the value of repeat boosters.

Some argue the goal of vaccination should be to prevent severe disease and deaths, not infections, so three shots may be sufficient for most people so long as efficacy against hospitalizations holds, says CMAJ article.

Others argue it makes more sense to focus on distributing vaccines globally and developing new variant-specific vaccines rather than repeatedly using the same shots in a few wealthy countries, with diminishing returns. The evidence to support four doses is limited.

So, what do you think? Is fourth dose necessary?

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