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An international group of scientists is arguing the average person doesn't need a COVID-19 booster yet, highlighting the intense scientific divide over the question.Jae C. Hong/The Associated Press

With 85 per cent of its eligible population vaccinated against COVID-19, Canada ranks as one of the best-protected countries in the world. For people who have already received their shots, this is welcome news, but it comes with an important caveat: The Delta variant is now dominant across the country and is driving case numbers upward. With cold weather approaching and driving people indoors, and the potential for other variants on the horizon, scientists and health authorities have turned their attention to the question of who may need additional shots to bolster their defences against the virus – and when.

Here’s a closer look at the science – and ethics – of getting a third dose.

Who in Canada is eligible for an additional dose of COVID-19 vaccine?

Last week the National Advisory Committee on Immunization (NACI) updated its guidance on additional doses of COVID-19 vaccines based on evidence that people who are moderately or severely immunocompromised may not mount an adequate immune response even after being fully vaccinated. NACI recommends that moderately to severely immunocompromised people 12 and older who have not yet been immunized should receive three doses of an authorized mRNA vaccine. Those in this group who have already been vaccinated, including those who received a combination of different vaccines, should be offered an additional dose of an authorized mRNA vaccine, the committee advised. This is not the same as receiving a booster shot, which is intended to restore initially adequate protection that may have waned over time.

While provinces can be expected to follow NACI’s guidance, Alberta, Ontario and Quebec have already been offering additional doses – not only to the immunocompromised but also to people travelling to countries that do not currently recognize their primary vaccine series. Alberta and Ontario also offer third doses to people living in higher-risk facilities such as long-term care homes. Saskatchewan has approved third doses for travel requirements, while B.C. announced Monday it would soon send invitations for third doses to the severely immunocompromised.

Vaccine studies

COVID-19 vaccine effectiveness against infection,

as a percentage, based on two separate studies

Zoe COVID study (Britain)

Effectiveness across time

1 month after second dose

88%

Pfizer-

BioNtech

5-6 months after second dose

74%

1 month after second dose

77%

AstraZeneca

5-6 months after second dose

67%

CDC study (U.S.)

Current effectiveness (June through Aug. 2021)

vs. the Delta variant

92%

Moderna

Pfizer-

BioNtech

77%

Janssen

(Johnson &

Johnson)

65%

THE GLOBE AND MAIL, SOURCE: zoe covid study; cdc

Vaccine studies

COVID-19 vaccine effectiveness against infection, as a percentage,

based on two separate studies

Zoe COVID study (Britain)

Effectiveness across time

1 month after second dose

88%

Pfizer-

BioNtech

5-6 months after second dose

74%

1 month after second dose

77%

AstraZeneca

5-6 months after second dose

67%

CDC study (U.S.)

Current effectiveness (June through Aug. 2021) vs. the Delta variant

Moderna

92%

Pfizer-

BioNtech

77%

Janssen

(Johnson &

Johnson)

65%

THE GLOBE AND MAIL, SOURCE: zoe covid study; cdc

Vaccine studies

COVID-19 vaccine effectiveness against infection, as a percentage,

based on two separate studies

Zoe COVID study (Britain)

Effectiveness across time

1 month after second dose

88%

Pfizer-

BioNtech

5-6 months after second dose

74%

1 month after second dose

77%

AstraZeneca

5-6 months after second dose

67%

CDC study (U.S.)

Current effectiveness (June through Aug. 2021) vs. the Delta variant

92%

Moderna

Pfizer-

BioNtech

77%

Janssen

(Johnson &

Johnson)

65%

THE GLOBE AND MAIL, SOURCE: zoe covid study; cdc

Which countries are currently giving third doses?

Israel was the first country to broadly offer third shots, making them available to people 60 and older in late July and quickly expanding eligibility to everyone 12 and older a month later. Hungary soon followed suit, offering them to the general population in early August.

Since then a number of other European countries, including France, Germany, Belgium, Austria, Ireland and Sweden, have begun to, or are set to, offer third doses to their more vulnerable populations. The U.K. is considering a fall booster campaign, beginning with more vulnerable populations, but has not yet announced its plans.

The United States intends to offer booster shots in the fall, pending authorization by the U.S. Food and Drug Administration (FDA) and recommendation by the Advisory Committee on Immunization Practices, a group within the Centers for Disease Control and Prevention (CDC). Individuals would become eligible 8 months after they received their second dose of an mRNA vaccine. The CDC currently only recommends an additional dose for moderately to severely immunocompromised people.

The U.S. Department of Health and Human Services said boosters will likely be needed for Johnson & Johnson’s one-shot viral vector vaccine. The department is awaiting data expected in the coming weeks.

What does science say about the need for additional doses in the general population?

This is currently a matter of debate. While some countries are making additional doses available, citing evidence of waning immunity, many experts say the evidence does not yet support such a move. In a commentary published Monday in the medical journal The Lancet, an international team of authors, including some affiliated with the FDA, point out the ways that observation studies of COVID-19 cases and outcomes could be painting a misleading picture of immunity. While they agree that booster shots may eventually be required, they say it’s too soon to pursue such a measure in the general population, particularly while it’s clear that the world would benefit far more from getting those doses to the unvaccinated.

How well are vaccines holding up so far?

Data continues to show the benefits of vaccination now that the more transmissible Delta variant is the principal driver of the pandemic in North America. In a report issued Friday by the CDC, an analysis of data gathered in 13 jurisdictions showed that once Delta became the most common variant, individuals who were not fully vaccinated had about a five times greater chance of being infected and were 10 times more likely to end up in hospital or die of a COVID-19 infection. The large study was based on more than 600,000 cases between April and mid-July. And while Delta is generating breakthrough cases among the vaccinated, the vaccines are still proving to be very effective against severe disease and death.

How do I know how protected I am as an individual?

This question remains one of the hardest to answer. What’s missing is what immunologists call a “correlate of protection” – a measurable factor that can reveal whether someone’s immune system is above the threshold that would allow them to fight off an infection. If such a measurement could be made reliably, it would provide a shortcut to assessing and comparing vaccines, including those that are still in development.

For now, researchers have mainly been looking at antibodies in the blood of vaccine recipients to see how well they neutralize the virus. This approach can offer clues to the extent and duration of immunity that vaccines provide, but it cannot absolutely differentiate between vaccinated individuals who will develop COVID-19 and those who won’t if exposed to the virus – or when.

What do vaccine manufacturers say about protection levels as time passes?

An ongoing analysis by Pfizer-BioNTech has shown a decline in the efficacy of its vaccine against symptomatic infection – from 95 per cent after the first two months to about 80 per cent four to six months after the second dose. The company said a booster significantly increased levels of neutralizing antibodies and that this data will be submitted to Health Canada and other regulatory authorities.

Moderna similarly found that antibody levels waned about six months after vaccination and that a third shot of half the original dose boosted antibodies to levels even higher than after the initial shots across all age groups. The company has submitted its data to the FDA and the European Medicines Agency.

AstraZeneca says an extended interval of as long as 45 weeks between the first and second doses of its vaccine resulted in a sharp increase in antibody response – as much as 18-fold – measured 28 days after the second dose. A third dose given at least six months after the second boosted antibody levels sixfold and increased neutralizing activity against variants, including Delta.

What determines when boosters will be needed?

If the immunity afforded by vaccines indeed begins to wane after several months, then a booster shot may become advisable for vaccinated Canadians by that time. The confounding factor is the Delta variant, which is now affecting immunity levels everywhere at once. Separating its impact from a loss of immunity due to the passage of time is a challenge. Another variable is the type of vaccine administered; for example, a different CDC study released Friday showed that vaccine effectiveness against the Delta variant is higher for the Moderna shot (92 per cent) than for the vaccine produced by Pfizer-BioNtech (77 per cent). This difference is echoed in antibody studies, including one by researchers at Toronto’s Mount Sinai Hospital. Bottom line: The jury is still out.

What are the ethical considerations of providing third and additional doses?

World Health Organization director-general Tedros Adhanom Ghebreyesus has called for a moratorium on booster shots until the end of the year, saying that while some at-risk populations may require additional doses, healthy, fully vaccinated people should not be receiving boosters. About 90 per cent of high-income countries have reached the WHO’s target of getting 10 per cent of population vaccinated, and 70 per cent have reached its 40-per-cent target, Dr. Tedros said. However, not a single low-income country has reached either target.

“Because manufacturers have prioritized, or been legally obliged, to fulfill bilateral deals with rich countries willing to pay top dollar, low-income countries have been deprived of the tools to protect their people,” he said at a news conference last week.

Kerry Bowman, a bioethicist at the University of Toronto, said Dr. Tedros’s call for a pause on booster shots is “very, very reasonable” and that vaccine supplies beyond what is required to adequately protect a population should go to lower-income countries.

“In the years ahead when the pandemic books are written, I think the greatest moral failure will be the global situation and how myopic many nations, including Canada, were during this pandemic,” Dr. Bowman said. “It’s an ethical problem and it’s an epidemiological problem.”

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