COVID-19

How effective is the COVID-19 vaccine dose?

LThe Canadian press posed these questions to Dr. Linore Saxinger, an infectious disease specialist at the University of Alberta, and Kelly Grindrod, an assistant professor at the University of Waterloo School of Pharmacy.

WHAT PROTECTION OFFERS THE FIRST DOSE OF THE TWO-DOSE VACCINE?

All approved vaccines have prevented hospitalizations and deaths due to COVID-19 in their clinical trials, and Dr. Saxinger says this is reflected in the real world.

Current data also shows that the first dose provides reliable protection with about 80% effectiveness against serious diseases within a month, she adds.

“This, of course, does not make you 100% protected, but most of the literature suggests that two to three weeks after the first dose, if you become infected with COVID-19, it will be more like a cold,” explains. Dr. Saxinger. “And converting COVID to the common cold is okay.”

WHAT HAPPENS TO THIS PROTECTION WHEN WE WAIT FOR A SECOND DOSE?

Experts say the protection provided by the first dose seems to improve over time, but they don’t know when it will start to wane.

Dr. Saxinger says mRNA vaccines from Pfizer-BioNTech and Moderna appear to provide better protection and at a faster pace, while the Oxford-AstraZeneca viral vector vaccine starts up more slowly but continues to grow.

Prof Grindrod notes that high levels of protection begin to appear about two weeks after the initial dose, and protection improves until full potential is reached with the first injection one month later.

“And then what we saw with the second dose is that it provides over 90% protection,” she says.

The National Advisory Committee on Immunization recommends that provinces defer the first and second doses for up to four months, depending on the proposal, to provide partial protection to more Canadians more quickly.

The recommendation raised concerns when it was first presented in March, but Ms Grindrod says the ideal spacing has never been set in stone. In clinical trials, doses have been split over three to four weeks, in part to get results more quickly, she adds.

Grindrod says there are different ways to measure long-term protection, including analyzing antibody levels and trying to capture T-cell responses. But she notes that the rate of hospitalization and breast death in the population is perhaps the best indicator that vaccines are at work.

“When we look at large populations (…) what is the hospitalization rate among vaccinated people versus unvaccinated people?” she asks. “We know you will have a pretty good response after this first dose, and you should maintain most of this benefit for at least a few months.”

WHAT HAPPENS IF I GET THE VIRUS WHEN MY BODY DEVELOPS IMMUNITY?

Dr. Saxinger says there have been cases of people contracting the new coronavirus within two weeks of the first dose of the vaccine.

She argues that this could mean that the person incubated the virus when he was vaccinated, or that his body did not develop enough immunity to recognize the pathogen and block it.

“You have to have time to make antibodies,” she explains.

Although Dr. Saxinger notes that the virus may be weakened by the immune response that occurs in the early days after vaccination, she says this is not fully understood.

CAN THE VACCINATED SEND THE VIRUS?

While some research suggests that people who have been vaccinated are also less likely to transmit the virus to others, Dr. Saxinger says there is still “not much conclusive evidence in this area.”

She argues that this is probably the main reason health officials are calling for caution, even among those who have already been vaccinated, as community transmission rates are skyrocketing in parts of Canada.

Prof Grindrod says Moderna’s data seems to suggest that those who have been infected after vaccination have lower viral loads.

“We really don’t know if these vaccines protect us from infection,” she says. “So, until most people are relatively protected from COVID, you still have the possibility of a potentially infectious infection.”

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