COVID cases among CHSLD residents not a reflection of vaccine effectiveness, experts say

·3 min read

Long-term care facilities in Montreal, Quebec City and Trois-Rivières have reported recent COVID-19 infections among residents who received the first dose of a coronavirus vaccine.

But medical experts and public health officials warn against reading too much into the situation, and say the fact some people have become sick since getting the vaccine is not a statement on its effectiveness.

"A vaccine doesn't provide immunity right the day you get it ... the first two weeks you're not protected," said Dr. Gaston De Serres, a medical epidemiologist at the Institut national de santé publique du Québec and a member of the province's vaccination advisory committee. "After two weeks, then, abruptly, the disease in vaccinated individuals pretty much stops."

The number of cases also remains small, relative to the number of people who have been vaccinated.

The Maimonides Geriatric Centre in Montreal, for instance, reported seven COVID-19 infections among its vaccinated residents this week. It appears there have been other cases in previous weeks as well.

But with a vaccine that clinical trials show confers roughly 50 per cent protection two weeks after a first dose (De Serres said his reading of the data reveals a much higher degree of immunity, in the order of 90 per cent) the expected number would be much higher.

Too early for firm conclusions

Even in a situation where 95 per cent efficacy is reached after a second dose, simple math suggests that in a population of 270 residents — which is roughly how many people have been inoculated at Maimonides — the statistical likelihood is 13 would not be protected.

As Montreal regional public health director Dr. Mylène Drouin told a news conference on Wednesday when asked about Maimonides, "it is a small number [from which] to draw a conclusion"

Similarly, the CHSLD Cloutier-du Rivage in the Mauricie region has found four infections among the 154 residents who have received the vaccine.

The CHSLD Saint-Antoine in Quebec City reported 66 infections among vaccinated patients on Dec. 30, two weeks after the residence and Maimonides became the first vaccination sites in Quebec. Though the numbers are considerably larger, it's too early for firm conclusions.

A spokesperson for the CIUSSS Capitale Nationale pointed out in a statement to CBC News the facility was in the midst of an outbreak that didn't manifest itself until a couple of days before the vaccinations began.

It's likely many residents were infected prior to receiving the vaccine. And that many others were exposed within the two-week window where protection is at its lowest ebb.

CBC News
CBC News

De Serres offered another possible explanation: during the clinical trials to establish the effectiveness of the Pfizer-BioNTech and Moderna vaccines, injections were given to subjects "who are generally younger and in better condition than people in long-term care facilities. Therefore, we may have, in this particular group of patients, a lesser protection."

That's among the many questions still outstanding as Quebec's vaccination program, which is using both the Pfizer and Moderna formulations, picks up steam.

Another is whether they will help curb asymptomatic transmission. The only certainty to this point, De Serres said, is "it protects against becoming sick, and it protects against becoming very sick."

Experts should soon have data from the CHSLDs that establishes how sick vaccinated patients got, and when it happened.

De Serres said it's not likely the vaccination program's effect will be noticeable in terms of deaths and infections among the elderly until mid-February, roughly a month after CHSLD residents and workers have received a first dose.

There are few certainties in medicine, and no vaccine is effective in every single case. But these, he said, are pretty close.

"I think the vaccine is really protective … it doesn't mean they are 100 per cent protective," said De Serres. "There are cases happening despite having one or even two doses. It's not because one is vaccinated that a person has to behave as if there is no risk."