Mixing doses of different Covid-19 vaccines, the first dose from another vaccine and the second from another, provides good protection against coronavirus, according to the very encouraging British study Com-Cov, the world's first to investigate efficacy of combinations of different vaccines.
The study alternatively considered two doses of Pfizer / BioNTech, two AstraZeneca (AZ) or a combination thereof (first one and then the other or vice versa). All combinations "worked" well, according to the BBC and the Guardian, which gives the health authorities flexibility in their vaccination plans.
Among other things, the study shows that those who have taken two doses of AZ may have an even stronger immune response if they take a third boost from another vaccine in the fall, if necessary. Some countries have already started mixing vaccines: Spain and Germany give mRNA vaccines (Pfizer or Moderna) as a second dose to younger people who have taken the first dose of AZ.
The Oxford University Com-Cov study, led by Professor Matthew Snape and conducted on 850 volunteers over the age of 50 (four weeks between doses), found that the combination of first-dose AZ and second-dose Pfizer produces more antibodies and T-lymphocytes than the reverse combination, i.e. first dose Pfizer-second AZ. Both of these combinations produce more antibodies than the two doses of AZ.
The first Pfizer-after AZ regimen produces almost seven times lower antibodies than two doses of Pfizer, but five times more antibodies than two doses of AZ. In summary, any combination Pfizer / Pfizer or AZ / Pfizer or Pfizer / AZ produces a stronger antibody and immune cell response than the AZ / AZ combination. Most antibodies are produced after two doses of Pfizer, while the strongest cellular immunity is from the combination of first dose AZ-second Pfizer. The first AZ-after Pfizer regimen is almost as effective as the two doses of Pfizer.
Another study shows that a third dose of AZ, given six months after the second, boosts the immune system. But experts say it's too early to know if people will really need an extra dose this fall, as it is still unclear how much vaccine immunity weakens over time.
Professor Paul Hunter of the University of East Anglia said: "The big question right now is whether boosts will be given in the autumn. "Based on the data so far, I suspect that this is possible for those who are more at risk of coronavirus, either because of age or because they are clinically vulnerable."
He suggested that those who have taken two doses of AZ could do so Pfizer as a booster dose in the fall, instead of continuing with AZ, while those who have taken two doses of Pfizer will probably not need a booster dose in the fall, based on the findings of the Com-CoV study.
It is recalled, however, that two doses of AZ have been shown to reduce the chance of severe Covid-19 and hospitalization by more than 90%, so the AZ vaccine is very effective. It is just slower, as it "builds" immunity in the body at a slower rate. However, if a third dose is needed, it appears that a boost with mRNA vaccine will provide even better protection.
On the other hand, according to the researchers, mixing vaccines seems to produce more short-term and mild side effects, such as chills, headaches and muscle aches.