Timing of second COVID-19 vaccine dose has importance: immunology professor

WINNIPEG — As the Manitoba government continues to open up eligibility for second-dose appointments for the COVID-19 vaccine, an immunology professor is explaining the importance when it comes to the timing of the second shot.

Dr. Jude Uzonna, an immunology professor at the University of Manitoba, said vaccines work on the concept of immunological memory. This is the immune system’s ability to respond more rapidly and effectively to pathogens it has previously encountered.

“So what happens is the first time you’re given a vaccine, it takes time for you to build a molecule that will protect you,” Uzonna said.

“It takes between 10, sometimes up to 30 days, for you to have those molecules appear. It’s just an equation of remembering.”

He said if someone gets their first and second doses of the COVID-19 vaccine too close together, it won’t be as beneficial.

“If you don’t wait for that time for the immune system to be able to learn how to do that. If you then go in the second time, you are not going to get that benefit of that memory, of remembering what you’ve done before and doing it perfectly in a shorter period of time and in a massive way that’s going to give you that maximum protection,” Uzonna said.

However, if someone waits too long to get their second vaccine dose, it could negate the immunological memory.

“Your immune system, what it has learned previously, how to do it. You risk, again, starting from scratch,” Uzonna said.

To get the maximum efficacy from a vaccine, Uzonna said people should wait between doses, but not too long.

He recommended that people listen to the clinical studies and pharmaceutical companies about the best time to get the second vaccine dose.

Currently, to book a second-dose appointment, eligible Manitobans must have received a Pfizer vaccine 21 days before their second-dose appointment or a Moderna vaccine 28 days before their second-dose appointment.


When it comes to a person getting a different type of vaccine for the second dose than they did for the first, Uzonna noted the studies are ongoing and there is no definitive answer as of yet.

He said there is some theoretical evidence that suggests mixing and matching vaccine could be beneficial.

“Number one: when you mix and match there’s evidence to show from previous studies of previous vaccinations that it actually may give you a stronger immune response,” Uzonna said.

He said this means the quality of the response, the molecules, and the antibodies a person produces will be higher.

“Second, it can actually enhance the breadth of the response,” Uzonna said, explaining this means the ability to neutralize more strains or variants of the virus.

“This is really because during mix and matching, you may actually induce both antibodies and T-cells.”

Currently, Manitobans are required to get the same type of COVID-19 vaccine for both doses.

– With files from CTV’s Nicole Dubé.

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