The reason why the AstraZeneca vaccine is not recommended for people over the age of 65 is not because it’s not safe, but because it was found to be less effective for seniors whose immune systems aren’t as strong.

A vaccine triggers the immune system, and Dr. Lisa Barrett, an Assistant Professor in the Department of Medicine at Dalhousie University—who specializes in Infectious Disease and Immunology—says clinical trials showed it was less effective in preventing serious illness in older patients.

She says infections are the leading cause of hospitalization for people over the age of 65. Dr. Barrett admits that there were some challenges with the AstraZeneca trials and until that’s “sorted out” the national body has not recommended using the vaccine in people over the age of 65.

In the meantime, the AstraZeneca vaccine is very safe and effective, says Dr. Barrett, and is easily distributed because it requires regular refrigeration.

She says if the choice is AstraZeneca or no vaccine, then the recommendation is not to withhold it.