COVID-19 modelling: Single vaccine dose prevented 70 per cent of infections among B.C. cohort

People are seen inside a restaurant and on an outdoor patio at English Bay, in Vancouver, B.C., on Monday, June 21, 2021. THE CANADIAN PRESS/Darryl Dyck

Vancouver -

A single dose of COVID-19 vaccine effectively prevented 70 per cent of infections in B.C. adults between the ages of 50 and 69, according to the preliminary results of a study conducted earlier this year in the province.

Provincial health officer Dr. Bonnie Henry shared the findings during her monthly modelling presentation on Monday, revealing that vaccines had a dramatic effect on infection numbers in that age cohort during the study period of April 4 to May 22.

The work was done using a method called test-negative design, which was developed at the B.C. Centre for Disease Control over several years to determine the effectiveness of flu vaccines.

"We now have the ability with the team at BCCDC to look at how well these vaccines are protecting us on a real-world basis here in B.C.," Henry said.

"We have overall a 70 per cent vaccine effectiveness and this is really good news."

The study took place after variants of concern – particularly the Alpha and Gamma variants – had already become dominant among B.C.'s COVID-19 caseload. The results still pointed to strong vaccine effectiveness, Henry noted.

"For all of the strains that we see circulating in B.C., the rate of infection in people who have been immunized is much, much smaller than those who have not yet been immunized," she said.

The approximately 60,000 participants involved were in their 50s or 60s, and had so far received just one shot of either the Pfizer-BioNTech, Moderna or AstraZeneca vaccines.

The results suggest the mRNA vaccines offered more protection, preventing about 75 per cent of infections. But the viral vector vaccines were still found to have prevented upwards of 60 per cent of possible cases.

As immunization numbers have climbed across B.C., case counts have plummeted. The province's rolling weekly average has dropped to 62 new COVID-19 infections per day – the lowest it's been since Aug. 14 – and Monday's modelling revealed the case rate has dropped to zero in dozens of local health areas.

Among them are Howe Sound, Hope, Quesnel, Nelson, Merritt, Golden and Nechako, none of which recorded a single case from June 13 to 18.

The data revealed that Surrey, which was the province's biggest COVID-19 hotspot for much of the pandemic, still has the highest raw number of COVID-19 cases, recording about 70 infections between June 16 and 22.

But Henry noted that the last time she presented modelling data, about one month earlier in May, "the scale was up in the 300s."

When measured in COVID-19 cases per capita, Surrey has also been surpassed by a number of much smaller communities, including Windermere, Fernie and Stikine.

B.C.'s latest dynamic compartmental modelling, which aims to chart how case numbers may surge or shrink under a number of different scenarios, indicates that the spread of the Delta variant may be one of the most influential factors in the coming months, along with the increase in social contacts.

While there could be an uptick in cases, officials expect it to be manageable.

"We will expect to see some cases again, and some clusters, and some transmission in certain situations – but it's not likely that we're to see that rapid increase in spread in the community the way we did even in March," Henry said.

"That's because we know we have a solid base of protection through immunization, and we have a strong public health program to follow up in each of the cases and clusters as they occur."

Health officials also indicated the province is on track to loosen more restrictions soon. The province set July 1 as an initial target for entering Step 3 of the government's restart plan, and Henry hinted the government will have "an announcement about that very soon."

Under the plans presented earlier this year, Step 3 promises a "return to usual for indoor or outdoor personal gatherings," increased capacity for organized gatherings that would allow for fairs and festivals, the lifting of party size limits and extension of liquor hours at restaurants, and the reopening of nightclubs and casinos with limited capacity.

The government has also suggested masks could become option as well, while remaining "recommended" in public indoor settings.