Mixing Covid vaccines is safe, but increases chance for side-effects

Adults taking two different coronavirus vaccine doses, such as one Pfizer and one AstraZeneca, may lead to an increased chance to suffer mild side effects but should not pose a real safety threat, preliminary clinical trials conducted in the UK have suggested.

The research has been conducted using both vaccines as a base and states chills, muscle pain, headaches and fever were reported more frequently when the vaccines were combined.

Read more: EU "will not" renew AstraZeneca vaccine contract

It also found it was more likely patients would take the day off work the next day after receiving the vaccine.

The research found that any other adverse effects were short-lived or negligible with no other safety concerns.

There have been concerns raised over whether or not mixing vaccine doses could help boost the immune system of patients or stand to cause side issues associated with the different vaccines.

Until this study, information had been relatively scarce in this field, although a number of industry bodies such as the US Centers for Disease Control and Prevention and Public Health England have warned against the mixing of vaccine unless absolutely necessary.

The Com-CoV study was launched in February in order to test this one way or the other, primarily to see if any benefits could be gauged from mixing vaccine doses or lead to scenarios where clinics could simply swap vaccines if supply fell short owing to significant bottlenecks hampering vaccine rollout worldwide.

The study was conducted on over 800 people over the age of 50. Full results are expected in June.

Both vaccine doses were delivered 28 days apart and participants were either given two AstraZeneca doses, one of each or two Pfizer doses.

The study was headed by the University of Oxford, which also helped develop the AstraZeneca vaccine.

Read more: Pfizer generates $3.5bn in Covid vaccine revenue as US drives demand

"These are the type of reactions you do expect with vaccines," said Dr Matthew Snape, the trial's overseer at a press briefing, adding it was an "intriguing finding," and not necessarily something "they expected to see."

He added: "They are more or less the same types of reactions that you're seeing with the standard schedules. It's just that they're occurring more frequently, and we're seeing both more frequent both in mild and moderate symptoms -- but they resolved quickly.

"One of the things it's telling us is that you wouldn't want to immunize a ward full of nurses on the same day with a mixed schedule because you may have higher rates of absenteeism the next day."

Feverishness was reported in 34% of the patients who received the AstraZeneca vaccine first and Pfizer as the second dose, seeing an increase from 10% in people who received two AstraZeneca doses.

Similarly, the rates of fever were up to 41% in those who received the Pfizer vaccine first, compared with 21% for those who received two doses of the Pfizer vaccine.

There were no hospitalizations owing to the study and most increased reactions were found within 48 hours, the research found.

The researchers also stipulated no cases of the blood clots linked with the AstraZeneca vaccine were discovered.

Read more: US chooses to waive intellectual property rights for vaccines

The Study was extended in April to contain 1,500 more volunteers and to allow them to test the Moderna and Novavax jabs alongside the Pfizer and AstraZeneca ones.

This will also give the researchers time to study the effects of immune responses based on different schedules and whether changes in these schedules cause stronger immune responses.

Back to Homepage

Back to Healthcare