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Mixing COVID-19 vaccines safe but effect on immunity not yet clear: U.K. study

Last Updated May 13, 2021 at 9:01 am PDT

A nurse reaches for a vial of AstraZeneca vaccine at a mass COVID-19 vaccination clinic in Calgary, Alta., Thursday, April 22, 2021.THE CANADIAN PRESS/Jeff McIntosh

Early study results suggest receiving a different vaccine as a second dose is safe

Preliminary results show mixing certain COVID-19 vaccines causes more frequent mild to moderate symptoms

B.C. doctor says still too early to make inferences on the effectiveness of mixing vaccines, but notes it's safe

VANCOUVER (NEWS 1130) – It’s not yet clear how it affects the immune system, but preliminary study results suggest mixing certain COVID-19 vaccines is safe, though it causes some more frequent symptoms.

This comes as more provinces, B.C. being the latest, decide to stop using the AstraZeneca shot as a first dose, reserving supply for second doses.

Researchers at Oxford University say there was an increase in short-term adverse reactions, with more frequent mild to moderate symptoms reported, after the Pfizer vaccine was followed four weeks later by AstraZeneca, or vice versa.

The ongoing study in the U.K. is now trying to determine how a combination of COVID-19 vaccines would affect the immune system’s response, compared with administering the same type of vaccine for both doses.

“It’s a first good step. It’s safe from what we can tell, but we just need to be kind of mindful when we plan for the rollout,” explained Dr. Birinder Narang, vice chair of the board of directors for the Burnaby Division of Family Practice.

“These are regular vaccine side effects, they’re short-lived. So what we found is that will happen, they’ll likely happen, but it’s not causing any significant problems,” he said of the symptoms.

Narang says there haven’t been any big safety concerns identified in the study so far, but he notes we can’t jump to any conclusions just yet.

“We can’t make any inferences on the effectiveness yet — that data is still going to be coming — but what we do know is it is a safe thing to try,” he told NEWS 1130. “We know that some people are getting more of the, kind of, regular symptoms that we expect to get from side effects whether it’s sore arm, fever, fatigue, or some gastrointestinal symptoms. So we know that has been identified — but we know that they’re mild and only last for a couple of days.”

He believes this can help us prepare and plan so that if we get to the point where people are offered a different vaccine for their second dose than the first one they received, we know what to expect.

Other symptoms reported after vaccination include chills, fatigue, headaches, and joint pain.

Chief investigator Matthew Snape, associate professor in pediatrics and vaccinology at Oxford, has said initial data are being released to inform people about symptoms as several countries consider mixing vaccines.

“The results from this study suggest that mixed dose schedules could result in an increase in work absences the day after immunization, and this is important to consider when planning immunization of health-care workers,” Snape said in a EurekAlert! statement issued by the service, which provides science-related releases.

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Researchers also noted that while the study participants were aged 50 and over, it’s possible that adverse reactions may be more prevalent in younger people, though they did not provide any details.

Some experts point out the small size of Oxford’s study means we won’t know the reactions some people could face when mixing vaccines.

Provinces across the country are keeping an eye on this U.K. study, including B.C., amid concerns over AstraZeneca.

B.C., Alberta, and Saskatchewan have already announced they would be pausing use of the AstraZeneca vaccine, saving remaining doses for second doses, citing supply as their reason for the decision.

Ontario has also suspended use of the shot. However, it has cited recent evidence of an increase in VITT blood clotting incidents as its reason, adding its decision was being made “out an abundance of caution.”

Oxford researchers are expected to release more information on the results of mixing doses in the coming months.

-With files from Robyn Crawford