2nd shot- does it matter which vaccine I get?
Health authorities in Canada and some European countries are now saying that when you go for your second COVID-19 vaccination, it does not really matter which vaccine you get. The recommendation is based on vaccine supplies – some products are more available than others – and concerns with the AstraZeneca vaccine, which has been linked to a rare type of blood clot called VITT. VITT, or vaccine-induced immune thrombotic thrombocytopenia, is an unusual complication because it results in platelets forming clots (thrombi) in people with a low number of platelets (thrombocytopenia) in their blood stream.
Mixing vaccine products is not usually an issue – no one thinks to ask who supplied their annual flu shot. But questions have been raised because the vaccine technology of the AstraZeneca vaccine (called viral vector) differs from that of the Pfizer and Moderna vaccines (called mRNA).
The Canadian recommendation to mix-and-match vaccines was issued in June by the National Advisory Committee on Immunization (NACI). After reviewing the evidence, NACI determined that getting a different vaccine – with the focus on getting the Pfizer vaccine after first receiving the AstraZeneca vaccine – was safe and effective.
So what is the evidence?
One study, called the CombivacS trial, is currently running in Spain. People who had received the AstraZeneca vaccine got a second shot at least 8 weeks later with the Pfizer vaccine. According to the preliminary results, people showed a very robust immune response to the booster. A separate analysis found that the booster effect kicked in about 7 days after getting the vaccination (Borobia et al. Lancet 2021, preprint 27 May 2021). So this suggests that the booster shot will be effective at preventing COVID – an important consideration as more contagious forms of the virus become more widespread.
Regarding the safety of mixing vaccines, the results differ depending on the study. A German study of 340 healthcare workers found that side effects were less common with the AstraZeneca/Pfizer combination (52%) compared to Pfizer/Pfizer (65%) (Hillus et al. medRxiv, epublished 2 June 2021). Severe systemic reactions were also less common with the mixed regimen. In the Spanish study, the typical vaccine side effects (e.g. headache, chills) after mixing vaccines were common but generally mild.
In contrast, in a U.K. study called CoM-Cov, side effects were more common after mixing vaccines. In that trial, 830 people received various combinations of the AstraZeneca and Pfizer vaccines either 28 days or 84 days apart (Shaw et al. Lancet 2021;397:2043-2046). With the AstraZeneca + Pfizer boost, the most common side effects were headache (65%), malaise (54%), chills (38%) and feverishness (34%). No serious side effects were reported.
So it appears that people who have received the AstraZeneca vaccine can safely get the Pfizer vaccine as their booster shot, although they may feel a bit poorly for a day or two afterward. NACI also said that it does not matter if one of the mRNA vaccines (Pfizer or Moderna) is substituted for the other.
U.S. regulators are recommending that people get the same vaccine for their booster shot. But this is largely because the AstraZeneca vaccine has not been approved there, and they have more than enough supply of the Pfizer and Moderna vaccines to go around.
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