With four approved COVID-19 vaccines in Canada, some have said they would be more hesitant to get the Johnson and Johnson shot, since its efficacy against mild to moderate COVID-19 was 72%, compared to the 95% efficacy Pfizer and Moderna boast.

But is it fair to compare efficacies?

Not only is it impractical to pass on an approved COVID-19 vaccine just because its efficacy was slightly lower than another vaccine, but doctors say it is also misguided. According to experts, the only way to fairly compare vaccines’ efficacy results head-to-head is to compare the vaccines head-to-head in a clinical trial.

“We should be cautious in comparing these trials since they used different ways to measure efficacy”, says Alison Thompson, an associate professor who studies public health policy and ethics at the University of Toronto.

“I think the Johnson and Johnson (Janssen) one had much better clinical endpoints in the trials … there was a pretty low bar for establishing efficacy in the other trials,” she says.

The Pfizer and Moderna trials looked at the prevention of any lab-confirmed infection starting at least seven to 14 days after the last vaccine dose in those who had symptoms. However, the Janssen trial set a higher bar, looking to prevent moderate to severe or critical infection. “As we get more data about the kind of efficacy that they have over the longer term, we may see those (efficacy) numbers come down significantly for the mRNA (Pfizer and Moderna) vaccines,” says Thompson.

The lower efficacy in the AstraZeneca and Janssen trials may also be explained by a greater number of infections from variants of concern, which emerged after the Pfizer and Moderna trials were completed and against which vaccine protection is expected to be lower. In the Janssen trial, for example, efficacy was 74 percent in the United States but dropped to 52 percent in South Africa, where almost 95 percent of cases were due to the B.1.351 variant. At least 136 cases of this variant had been detected in Canada as of March 8.

Comparisons should also take into account differences in the number and timing of doses. However, the National Advisory Committee on Immunization expects that short-term efficacy will likely remain high, even with an extended four-month interval between doses.

To be clear: Doctors consider all four available vaccines to have high efficacy and to be safe and say, from a practical standpoint, people should get whatever shot is available to them first.