Targeted vaccination helped flatten COVID’s third wave: Montreal study

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TO recent study by the Montreal public health department suggests using targeted vaccination in COVID-19 hot spots early can help slow transmission — a lesson the authors hope will be considered during future waves of COVID-19 or in the event of another pandemic.

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While Quebec’s mass vaccination strategy used a distribution approach aimed at reducing deaths among the vulnerable, Montreal public health set out to see, in March and April 2021, if “ring vaccination” could be used in tandem to help limit the spread of the Alpha variant and reduce the impact of the third wave in the city.

The study published in JAMA last week — the first of its kind for COVID-19 — suggests the intervention did just that.

“With the local data we had on the epidemiology in Montreal, where we could really see the arrival of the new variant in very specific neighbourhoods, age groups and communities… We clearly saw that our epidemiology had the characteristics that allowed us to experiment with this strategy,” Montreal public health director Dr. Mylène Drouin told the Montreal Gazette.

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The intervention targeted parents and teachers from dozens of schools and daycare centers in Montreal neighborhoods hardest hit by the variant. Nearly 12,000 people who were not yet eligible according to Quebec’s criteria were given a first dose of mRNA vaccine in parts of Côte-St-Luc, Outremont, Plamondon and Snowdon.

“Transmission was very much within families, with students in daycares, schools,” Drouin said. “It was really in families with multiple children (sometimes at different schools) … That’s when we decided to vaccinate the parents.”

Dr. Geneviève Cadieux, another co-author of the study from Montreal public health, explained that when it came time to evaluate the impact of the intervention — the initial goal was simply to control transmission — they created population groups “that we could then compare between each other and in time.”

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Those groups were the primary intervention group, located in hot spots; a secondary group, located on the periphery who interacted with schools in the hot spots; and the rest of the Montreal population not targeted by the strategy.

“We compared them before the intervention, during the intervention and after the intervention,” Cadieux said. “About three weeks after the intervention period, the incidence rate (in the primary intervention group) came back to about what it was in the general population of Montreal.”

Cadieux said the group knew the strategy had been successful for other airborne diseases, such as the measles, and that there had been modeling for COVID-19, “but there was no one who had tested it on the ground.”

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“If it works for something that is as transmissible and airborne, it should work in theory with COVID, and so that’s what we basically showed,” she said. “It would take, in theory, a randomized clinical trial to prove it for sure — but our data strongly suggests it’s a strategy that works for COVID.”

Armed with the knowledge that the strategy could help, the team attempted to see if the same method could be applied as the Omicron variant took hold in the city in December, but ultimately discovered the characteristics of the outbreaks weren’t favourable.

“The conditions required are that the variant is geographically concentrated or concentrated in a specific population or subpopulation,” Cadieux said. “We never really identified one population to target — the first cases of Omicron came from everywhere.”

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If the team had had its way, it would have continued targeting hot-spot neighborhoods in Montreal beyond the initial experiment, but it wasn’t allocated the doses to stray from eligible age groups. Drouin said the province, which approved the intervention on the condition that it be evaluated, was pleasantly surprised by the results.

The provincial health ministry, for its part, said in an email to the Montreal Gazette that while it commends the work of local health authorities “and is pleased that they publish their experiences in their field and share what they’ve learned,” it is “far too early to comment” on the strategy by Montreal public health.

The hope now, Drouin said, is that the data will encourage taking local epidemiology and various intervention strategies into account moving forward.

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“The idea is to say that it’s a strategy to keep in our tool box when the epidemiological conditions allow it,” she said. “If we end up having a new vaccine that covers all coronaviruses and we don’t have a lot of them, well maybe we could reflect on integrating this strategy in our reflection of the criteria for distribution.”

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