The UK started a trial in February to see if combining two different vaccines works. Experts suggest that it could be a valid alternative.

A toilet opening a box of AstraZeneca vaccines.

The AstraZeneca vaccine continues to give serious headaches to health authorities across Europe. After the EMA concluded this Wednesday that cerebral venous sinus thrombi could have “possible links” with the drug and that these unusual blood clots should be included as “very rare side effects”, the communities and Health agreed to suspend vaccination in those under 60 years of age (most of them essential personnel such as police officers and teachers).

Now, Health has a new open front to resolve: what is going to happen now with those people who have already been vaccinated with the first dose. Will they get a second dose of AstraZeneca? Will the Pfizer or Moderna inoculate be used in the second injection, or will these people be left with the first prick instead? According to the Minister of Health Carolina Darias said yesterday, it is about a question that “is yet to be determined” and that will be resolved in the coming weeks. Darias, in any case, yesterday cited two possibilities: administer a second different vaccine to AstraZeneca or leave people who have already been vaccinated with this drug with only the first dose.

The third scenario that could be contemplated, and which does not rule out Health either, is to administer the second dose of AstraZeneca to those people who have already received the first regimen (about 2.1 million people, according to Health data). But what could happen in each of these scenarios?

The second dose of Pfizer?

The United Kingdom Government launched a clinical trial last February with 800 people aged 50 and over to find out if the combination of two different vaccines (Pfizer, which uses the mRNA technique; and AstraZeneca, which uses a adenovirus) works as well as the current approach. As Cristian Smerdou, director of the Gene Therapy and Regulation of Gene Expression Program at the University of Navarra, explains to EL ESPAÑOL, could be the best strategy as long as it was previously approved by the European Medicines Agency (EMA).

The AstraZeneca vaccine is a drug that is based on an adenovirus that infects the cells of our body and that requires two doses, a main and a booster that is used to stimulate the memory cells of the immune system and amplify this response. “When the first dose is injected, this virus expresses the gene for the coronavirus spike protein and an immune response is produced against the gene that carries it. Thus, when you inject the second dose, more widely spaced, this response has already been generated and could favor the efficacy of the first dose, ”says Smerdou.

As the expert says, previous experiences suggest that mixing vaccines could be a good strategy. In fact, some Ebola immunization programs have mixed different injections and shown that protection is improved. “The ‘limitation’ of AstraZeneca is that two doses of the same adenovirus are givenThis is not the case with the Sputnik V vaccine, which uses an adenovirus 26 and an adenovirus 5 from humans ”, explains Fernando Moraga-Llop, vice president of the Spanish Association of Vaccination. The problem is that, as Moraga-Llop indicates, the development of this strategy implies the completion of the trial in the United Kingdom, which lasts for several months and the subsequent approval of the EMA, so it seems implausible.

What if they leave me with just one dose?

The other scenario that Health contemplates is to do nothing with the 2.1 million vaccinated with the first dose of AstraZeneca. It is the option to be followed in the United Kingdom. Where would this strategy leave this population group? As Darias has explained, the Oxford vaccine is more than 76% effective against Covid-19 with the first dose. However, what would happen next to immunity is unknown. According to a study published in the journal The Lancet, the efficacy of the vaccine falls below 35% after three months.

What about immunity? “It may be that the memory response is shorter without giving the second dose and that the immune response decreases earlier. Surely, giving a booster dose will promote protection, which does not mean that it is infallible to protect against serious disease because there can always be some case, “says Smerdou. “The second dose is known as ‘buster’, a booster or booster dose and we do not know what will happen because no clinical trial has evaluated the one-dose schedule and we would not be complying with the schedule set by the technical sheet or with the EMA” , says the vice president of the AEV.

Put another dose of AstraZeneca

Considering that cerebral venous sinus thrombus events linked to the AstraZeneca vaccine are extremely rare (just over 30 cases have been detected in over 18 million vaccinated in the UK), experts also raise the possibility of give the second dose to those who have already received the first and they didn’t have any complications. Would this imply a higher risk? In principle, as explained by Federico Martinón-Torres, member of the WHO Vaccine Advisory Committee and researcher at the Hospital Clínico Universitario de Santiago, it would not have to.

“Although it has not yet been identified, when such a rare adverse effect occurs with a drug, in general there is a specific predisposing factor in that person. So if there were no problems with the first dose, it is very unlikely that you will have it with the second dose, “says the specialist. “If you really had that greater predisposition, it would have already manifested.”

Moraga-Llop insists that, despite the fact that vaccination with AstraZeneca has been suspended in those under 60 years of age, “The benefit-risk balance is favorable to the benefit”. Why then in the United Kingdom has vaccination been limited to those under 30 years of age? This is a measure that makes sense if we take into account that in this population the risk may be somewhat greater than the benefit because this population does not have as much risk of becoming seriously ill. “In any case, the ideal is that all countries go in unison with the recommendations of the EMA and the rest of the health authorities. What would be unfortunate is that three communities came out that did one thing and another three that did another ”, he concludes.