What level of vaccination will it take for tide to turn? Experts weigh in

Article content

As Quebec’s COVID-19 vaccination campaign picks up speed, with more than 30 per cent of Quebec adults having received at least one shot, many wonder when all these jabs and safety measures will start to turn the tide.

As of 11 a.m. Thursday, almost 2.6 million doses of vaccine had been administered across the province, 84,837 of them in the last 24 hours alone; a single-day record. Another 3,856 people got their second dose of vaccine, bringing the tally of fully vaccinated Quebecers to 41,299.

Premier François Legault has set a target of getting a first dose to 75 per cent of the adult population by June 24. That’s over 5.2 million doses. So far, 2,546,004 doses have been administered, with 640,576 appointments pending. We are getting there.

But some experts have said we won’t achieve “herd immunity” — that is, enough people with antibodies to the virus that even those who are not immune are protected — until we have vaccinated 85 or even 90 per cent of the population.

Article content

But recent data out of the United Kingdom and Israel suggest that case numbers can drop substantially, and life can resume some normalcy, at much lower levels of vaccination. So, what proportion of the population has to roll up their sleeves here for that to happen?

“It’s a great question and the simple answer is we don’t know yet,” Dr. Timothy Evans, executive director of the federal COVID-19 Immunity Task Force told the Montreal Gazette. “But there are at least three things that are encouraging.”

First is the data from Israel, he said, where new cases have dropped from a peak of almost 10,000 a day to about 140. Life there is almost back to normal, with restaurants, bars and hotels open to anyone who can prove they’ve been vaccinated, and no more obligation to wear masks outside. About 55 per cent of Israel’s adult population has been vaccinated. When those who were previously infected are included in the count, it is estimated that more than 60 per cent of the population now has some level of immunity to COVID-19.

Article content

“That to me is a very encouraging sign that the threshold for decreasing new infections or keeping the reproductive number for viral transmission below one, may not be as high as we thought,” Evans said.

The second “very encouraging” piece of information comes from the United Kingdom, a country that has taken a similar approach to Quebec’s, in delaying the second shot in order to get more people protected with a first jab. In the U.K., just over 50 per cent of the population has had one shot, while another 10 per cent is fully vaccinated. In terms of new cases, “we see a very similar trend as in Israel,” Evans said.

“You don’t see any evidence there (the U.K.) of a third wave at the moment. And that’s a big difference with respect to the rest of Europe, where vaccination rates are much slower.”

Article content

But Evans points to a two big caveats. First, the level of “background immunity” due to previous infection in both the U.K. and in Israel are quite a bit higher than in Canada.

Serology studies done through blood banks here after the first wave showed that between one and three per cent of Canadians (2.3 per cent of Quebecers) had developed immunity through infection.

“So the vast majority of Canadians had not had infection after the first wave. In the second wave, we don’t have the definitive data yet, but it looks as if it may still be in the order of single digits for Canada as a whole in terms of background immunity.”

“This is important because if you have a big part, say 25 per cent, of your population that has infection-acquired immunity, then that will help slow the spread of infection as well as the vaccines.” So vaccination rates would have to be higher in Canada to see similar drops in new cases, he said.

Article content

The other caveat is the variants.

Dr. Brian Ward, professor of Infectious Diseases at the Research Institute of the McGill University Health Centre, agreed with Evans that estimating the vaccination level at which we can expect new cases to start to decline significantly is tricky.

“The ‘threshold’ for impact … is a moving goalpost,” he said. “There are four major wild cards in any calculation: transmissibility of the virus and its variants, level of protection provided by past infection and vaccination, the number of contacts that most people have per day, and personal protection measures.”

If Quebecers continue to keep daily contact relatively low and keep up a reasonable level of personal protection, then “even a modest level of protection from vaccination (or) prior infection can have significant impact,” Ward said.

Article content

Barring the arrival of new variants that can evade current vaccines and natural immunity, or are more transmissible or virulent, Ward says. “most of us believe that herd immunity might start to have real impact at something between 60 to 70 per cent of the whole population (including children).”

The take-away, of course, is that anyone who is eligible to be vaccinated should be rushing to do so, Evans said.

The good news on that front is that Quebec’s latest numbers seem to indicate that younger people seem much less concerned about taking the AstraZeneca vaccine than those 55 and older. More people in the 45 to 54 age group (329,316 people) got the AstraZeneca shot or made an appointment to do so in the one day that they have been eligible than did those in the 55 to 59 age group in the over two weeks they have been eligible (312,621 people aged 55 to 59 have gotten shots or made appointments since April 8).

Article content

Some younger Twitter users have been pointing gentle fun at their elders, for appearing to hold out for a “safer” vaccine. Hashtags like #GenX and #AstraZeneca promote generational bonding over the willingness to get the shot.

“After 13 months of masking, working from home, parenting through virtual school and stay-at-home orders, #GenX says “Give us ALL the AstraZeneca,” tweeted one happy GenXer.

AstraZeneca has been linked to blood clots in very rare cases, although the risk of blood clots from COVID-19 is much higher. While AstraZeneca is considered slightly less effective at preventing infection than Pfizer or Moderna, it is proving to be more effective than those vaccines at preventing hospitalization and death from COVID-19.

mlalonde@postmedia.com

All our coronavirus-related news can always be found at montrealgazette.com/tag/coronavirus.

Sign up for our email newsletter dedicated to local COVID-19 coverage at montrealgazette.com/coronavirusnews.

Help support our local journalism by subscribing to the Montreal Gazette here.