Over the next month, 400,000 British Columbians are expected to be vaccinated as the province begins one of the largest mass immunization efforts in its history.
B.C.’s vaccination strategy, according to provincial health officer Dr. Bonnie Henry, is based on the principal of providing the “greatest benefit to our whole community.” From that has flowed several key decisions about who gets the vaccine, and when, as well as the timing for second doses.
Vaccine efficacy is a complex subject, but Henry has tried to break it down in recent briefings. Here are some things to know:
Mind the gap
On Tuesday, Henry emphasized B.C. is following the “science of vaccines” in its decision to extend the gap between first and second doses to 112 days, which will mean all eligible adults should be able to get their first dose by the end of July, rather than September.
“Our focus is on maximizing the number of people who are benefiting from that very high, real-world protection that we’re seeing from a first dose of vaccine in B.C.,” she said.
Research done by the B.C. Centre for Disease Control was used to make the decision to extend the gap, said Henry, explaining that the original clinical trials for the vaccines were focused on getting information about whether they worked and were safe. “That’s why they used as short of an interval as they could possibly use, in most cases three to four weeks.”
B.C. researchers led by Dr. Danuta Skowronski and Dr. Monika Naus have looked at how the vaccines performed in real-word conditions.
Their findings were encouraging. “The data tells us … the benefit and the protection from a single dose, after about three weeks, is upwards of 90 per cent,” said Henry. “That lasts for a long time, at least for several months.”
Do I have a choice?
If you are receiving a shot at one of B.C.’s mass vaccination clinics, you won’t have a choice between the Pfizer or Moderna vaccine.
“You’ll take the one that’s there in front of you, and I encourage everyone to take the one there in front of you,” said Henry.
When it comes to the recently approved AstraZeneca vaccine, which is about 62 per cent effective after the second dose, there may be some choice.
Because B.C.’s mass vaccination program was built on the availability of the Pfizer and Moderna vaccines, the AstraZeneca vaccine will be offered strategically to targeted groups where it will have the greatest impact, which includes first responders as well as those who work in essential workplaces where there have been outbreaks in the past, said Henry.
“So there is a little bit of choice,” she said. “I would not suggest people wait. These vaccines are all good and they all work, and we’ve seen that across the world.”
Efficacy versus effectiveness
Efficacy is a measure of how much a vaccine lowers risk of a certain outcome, whether that be serious illness, hospitalization or death. In clinical trials, scientists measured how many vaccinated people still got COVID-19, compared to how many people got COVID-19 after taking a placebo. The difference in risk is calculated as a percentage.
Several factors impact efficacy, including where a clinical trial took place and if variants were established in those places.
Once a vaccine is in use, its effectiveness can be determined. Effectiveness is a real-world measure, meaning how effective the vaccine is on people.
Henry said B.C. scientists “have a lot of experience in this” from looking at the effectiveness of the province’s immunization programs, including for the seasonal flu. The effectiveness of the COVID-19 vaccines has proven to be very high.
As more people are vaccinated, B.C. should begin to see the “vaccine multiplier effect.”
“The more people we have immunized in our community, the less probability that the virus finds somebody it can transmit to and pass on,” said Henry. “That protects all of us. It magnifies the effect of an individual protection. We know the positive impact of this goes further than even every individual who is vaccinated. We know that the risk of passing the virus onto others is dramatically decreased. … It’s added protection to all of us, to our communities, our families, and everybody that we’re around.”
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