Towards a possible mix of vaccines but the knot is not solved

The hypothesis of a 'mix' of anti-Covid vaccines, using two different immunizers for the first and second dose, becomes more concrete and would allow greater flexibility of the vaccination campaign precisely in the phase in which the aim is to extend vaccinations to the entire population. The Commissioner for Emergency Francesco Paolo Figliuolo opens up to this option, but the knot is not yet resolved because, although it is an "interesting" option also according to the Italian Medicines Agency (Aifa), the scientific data on the 'combinations vaccines' are limited and not definitive to date and the few studies carried out have not yet been published.

"There are advanced studies on the second heterologous dose, ie doing the first dose with Astrazeneca and the second with Pfizer or Moderna, and they seem to give an excellent response," Figliuolo noted. According to the Spanish Combivacs study, not yet published, the heterologous vaccination program - that is, with two different vaccines - is safe and effective. Combivacs enrolled 670 participants aged 18 to 59 who already received a dose of AstraZeneca: Those who received a second dose of Pfizer vaccine, 450 subjects, showed a 150-fold increase in antibody titers, which remained evident 7 days after administration. However, the same authors point out that more data from larger studies are needed to confirm immune responses.Another study whose preliminary data have just been published in Lancet - the English research called Com-COV on 830 subjects over-50 - has instead shown that the mix with a first dose of AstraZeneca and the second with Pfizer causes an increase in reactions mild adverse events such as fever, fatigue, headache and pain, with "some short-term disadvantages" mitigated with the use of paracetamol. These side effects were reported in 34% of the sample, compared to 1.7% in the Spanish study.mitigated with the use of paracetamol. These side effects were reported in 34% of the sample, compared to 1.7% in the Spanish study.mitigated with the use of paracetamol. These side effects were reported in 34% of the sample, compared to 1.7% in the Spanish study.

However, Lancet notes at the same time, "it is reassuring that these symptoms were short-lived and that there were no concerns from the limited haematological and biochemical data available" relating to rare thrombosis. On the other hand, data on immunogenicity and antibody response are not available, expected by June. Also in Italy, at the Spallanzani Institute in Rome, a trial on the second dose started last April, after the first with AstraZeneca, using other vaccines including Sputnik. In short, the question, from a scientific point of view, is still open as noted by the president of the Technical Scientific Commission of Aifa Patrizia Popoli.

The vaccine mix, he says, is "an interesting option because it would support the flexibility of the vaccination campaign and would still allow full vaccination to continue in subjects who have already received the first dose of AstraZeneca but in whom it may be more appropriate to use another vaccine. However, if, to date, there is the hypothesis of an encouraging effect, there are still no definitive scientific data ". The British data on the increased occurrence of mild side effects with the use of the vaccine mix, for example, "could be associated with a greater immune response, but this - specifies Popoli - has to be demonstrated". Waiting for other data, the virologist Fabrizio Pregliasco also evaluates the mix hypothesis with interest. Previously, he notes, "the mix worked for other types of vaccines, for example against hepatitis. In the case of Covid-19, two different vaccines would stimulate different sites of the Spike protein of the virus and this should lead to greater protection ".

Meanwhile, some countries have already decided to adopt the vaccine mix solution: after Finland, Canada has now established that those who have received AstraZeneca can use Pfizer or Moderna for the second dose. Similarly, immunizations with Pfizer and Moderna (mRna vaccines) can be alternated between first and second doses. However, Canadian authorities point out that it would be 'optimal' to use the same vaccine for both doses. (HANDLE).