Covid-19: mixing vaccines is “safe and effective”

Mixing vaccines does not seem to be a problem and would even be “safe and effective”, according to the results of a study conducted in Spain. In preliminary results presented last Tuesday and also shared by the scientific journal Nature, the Carlos III Health Institute (ISCIII), which depends on the Spanish Ministry of Health, wants to be reassuring when two million Spaniards should be affected by a combination of vaccines. “Those who have received a first dose of AstraZeneca can safely and effectively receive a second dose of Pfizer”, affirmed Jesús Antonio Frías who coordinated the test Combivacs who studied the combined administration of AstraZeneca and Pfizer.

More neutralizing antibodies

In France, receiving two injections of two different products concerns people under 55 who have received AstraZeneca as the first dose. For these patients, the second dose should contain a messenger RNA vaccine, Pfizer or Moderna. This recommendation of the High Authority for Health (HAS) dates from April 9 and follows the rise of cases of thrombosis in several people. The use of the AstraZeneca vaccine was suspended for a time before resuming with an asterisk: this product is now reserved for people over 55. In the meantime, more than 500,000 patients under the age of 55 who received it as the first dose must therefore receive Pfizer or Moderna as the second dose.

For this study, the researchers compared the immune response in 600 patients. Among them, 442 received the Pfizer in the second injection after having received the AstraZeneca in the first and 221 had only the first dose. The cocktail of vaccines in the first group generated a number of antibodies “very satisfactory” fourteen days after the second injection welcomed the researchers. “Regarding the starting level, after the second dose of Pfizer, the antibody titers (the amount of antibodies) IgG increases up to 150 times and that of neutralizing antibodies increases about seven times”, noted Teresa Pérez, the coordinator of the serology laboratory of the Carlos III Health Institute.

No serious side effects

Although this study did not compare vaccination with two doses of the same vaccine, the researchers believe that the results show a better immune response after the combination of two vaccines. The level of antibodies “is higher than that found in the literature”, assures José Alcamí, researcher at ISCIII, who shows that the number of neutralizing antibodies increases approximately three times with the second dose of AstraZeneca and five or six times for the double dose of Pfizer.

Furthermore, no side effects led to additional medical care or hospitalization. The onset of pain at the site of infection (88.2%) is the most reported adverse effect. About half of the participants suffered from headache (44.4%) and general malaise (41.7%) and in very rare cases, only 2.7%, fever developed. But virtually all symptoms were gone by the third day after infection. “The data is very similar to the Pfizer trial”, noted Magdalena Campins, head of the epidemiology department and coordinator of the trial at the Vall D’Hebron hospital in Barcelona.

Results that are confirmed

These results confirm those of a British study whose data were published a few days earlier in the journal The Lancet. This highlighted an increase in mild and moderate reactions but without serious safety issues after combination vaccines. Furthermore, the results indicate that the symptoms do not last and resolve within 48 hours. “There were no hospitalizations due to solicited symptoms, and most of this increase in reactogenicity (excessive immunological response to a vaccine, editor’s note) was observed within 48 hours of immunization.“, wanted to specify the researchers.