Mixing Pfizer and AstraZeneca Covid vaccines increases mild side effects, early results show

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  • The Oxford study testing the mix-and-matching of Covid vaccines has released its first preliminary results
  • Some participants who received mixed doses had more mild to moderate symptoms compared to those who were given two doses of the same vaccine
  • No safety concerns were reported among the group

People who receive the Covid vaccine by Pfizer followed by the AstraZeneca jab – or vice versa – are more likely to experience an increase in mild to moderate symptoms from the shots, compared to people who receive two doses of the same vaccine.

This is based on the preliminary results of the first study to test the effectiveness of mix-and-match vaccines, also known as "heterologous dosing". The study is being conducted by researchers at the University of Oxford and the National Immunisation Schedule Evaluation Consortium (Nisec). It was launched in February 2021, News24 previously reported.

According to the results, chills, headaches, feverishness, and muscle pain were reported more frequently among participants who received the mixed doses. However, any adverse reactions were short-lived and no other safety concerns were noted, wrote the researchers.

The researchers have yet to determine the impact of these mixed schedules on immunogenicity – in other words, how effectively the vaccine doses induce an immune response in people – and said data on this will follow in the coming months. 

They also stressed that since the data is based on volunteers aged 50 and above, there is a possibility that these reactions may be more evident in younger age groups.

The results were published in The Lancet last week.

Potential effect on absence from work

“Whilst this is a secondary part of what we are trying to explore through these studies, it is important that we inform people about these data, especially as these mixed-doses schedules are being considered in several countries,” Matthew Snape, associate professor of paediatrics and vaccinology at the University of Oxford, and chief investigator of the trial, said in a news release

Snape added that the results suggest that mixed dose schedules could result in an increase in work absences the day after vaccination, which is important to consider when planning to inoculate healthcare workers.

"One thing it's telling us is that you wouldn't want to vaccinate a ward full of nurses [on a mixed-dose schedule] on the same day, because you might have more absenteeism the next day,” he told BBC News.

The trial, known as the Covid-19 Heterologous Prime Boost study or "Com-Cov" study, will run for a year and has already recruited over 800 participants over the age of 50 in England. 

Why mix and match?

The concept of mix-and-matching vaccines is nothing new and has previously been used in a clinical trial for an Ebola vaccine approved in May 2020, notes an article in The Conversation

In the context of Covid, pairing the doses from two different vaccines might give individuals longer-lasting immunity; produce a stronger immune response and better protection against the highly infectious variants that have and continue to spread rapidly across the world; or simplify vaccine rollouts, such as in a case of unforeseen vaccine shortages or sudden concerns about side effects. 

"If we do show that these vaccines can be used interchangeably in the same schedule, this will greatly increase the flexibility of vaccine delivery and could provide clues as to how to increase the breadth of protection against new virus strains," Snape previously said.

The authors indicated that further studies will evaluate the pairing of the Covid vaccines by Moderna and Novavax. According to BBC News, 1 050 participants were added to the study in April for the purpose of testing the pairing of these two vaccines. 

“[These additional trials] are crucial to informing the appropriateness of mixed Covid-19 vaccine schedules,” the authors wrote.

*For more Covid-19 research, science and news, click here. You can also sign up for our Daily Dose newsletter here.

READ | Covid-19: Oxford trial to test efficacy of mix-and-match vaccines

READ | WHO to restart Covid trial to test if three existing drugs can save lives of hospitalised patients

READ | Covid-19: A potentially deadly ‘black fungus’ infection seen in patients in India

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