A new trial in the UK is set to test the efficacy of giving people a “mix-and-match” of COVID-19 vaccines, using the first dose of one vaccine brand and the second dose of a different brand. The project seeks to answer the burning question of whether it’s safe and effective to use combinations of COVID-19 vaccines in the event of short supplies.
The COVID-19 Heterologous Prime Boost study, or "Com-Cov", run by the National Immunisation Schedule Evaluation Consortium, will recruit 820 people over the age of 50 who have not yet had a vaccine from eight different sites across the UK.
Four different combinations of two vaccines will be tested: a first dose of the Oxford-AstraZeneca vaccine followed by boosting with the Pfizer vaccine; a first dose of the Pfizer vaccine followed by boosting with either the Oxford-AstraZeneca vaccine; a double Oxford-AstraZeneca vaccine; or a double Pfizer vaccine. All four of these combinations will be evaluated with two different dosing schedules: at a 4-week interval to gather early interim data and at a 12-week interval. Blood samples will be taken over time to measure the patient’s antibody response and any adverse reactions will be noted.
“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,” Matthew Snape, Associate Professor in Paediatrics and Vaccinology at the University of Oxford and Chief Investigator on the trial, said in a statement.
The UK came under some heat in early January 2021 after the New York Times published an article claiming the UK had already updated its policy to allow “mix-and-match" vaccinations, "worrying experts". The Editor of the British Medical Journal criticized the article for being sensationalist and misleading as the UK's official guidance from the Joint Committee on Vaccination and Immunisation states anyone already given the Pfizer-BioNTech or Oxford-AstraZeneca jab in the UK should get the same vaccine for both doses. However, it did raise the question of whether this mix-and-match idea had yet undergone the necessary scientific testing to prove it was safe and effective.
While the US Centers for Disease Control and Prevention has said that authorized COVID-19 vaccines in the US “are not interchangeable,” Public Health England (PHE) has maintained that different vaccines can be mixed in "extremely rare" circumstances. Dr Mary Ramsay, PHE's head of immunizations, told BBC News where the brand of an initial jab is unknown or the same one not available, it is "better to give a second dose of another vaccine than not at all." The new trial hopes to settle some of these concerns and show whether a combination of different vaccines is a safe and effective means to protect against the infection.
In fact, some scientists are hopeful that a combination of different vaccines might actually prove to be even more effective than sticking to one thanks to something known as heterologous boosting.
“We have seen that the Sputnik vaccine uses a form of heterologous boosting – it is a vector vaccine, and the vector is different for the two doses. This seems to have been done partly out of concern that the immune system might recognize the vector virus used, reducing the efficacy of the booster dose,” explained Dr Peter English, Consultant in Communicable Disease Control and Immediate past Chair of the BMA Public Health Medicine Committee.
"Given the inevitable challenges of immunising large numbers of the population against COVID-19 and potential global supply constraints, there are definite advantages to having data that could support a more flexible immunisation programme, if needed and if approved by the medicines regulator," Deputy Chief Medical Officer and Senior Responsible Officer for the new study Professor Jonathan Van-Tam said.
"This study will give us greater insight into how we can use vaccines to stay on top of this nasty disease."
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