It may have happened that you wished to get a specific vaccine but ended up getting another one — may be because a particular vaccine was available. I have also come across a lot of people who have said, "I was waiting for Moderna or Sputnik, but then got Covisheild because it was available."

However, one should not undermine the gravity of vaccine unavailability. Though considering the situation we are in right now, everyone must get a vaccine without being picky. Especially with vaccine drives being made available in housing societies and offices, taking whichever vaccine is readily available should be a priority.

But what happens if the vaccine you have taken becomes unavailable for the next shot? The thought has crossed each of our minds at least once. Surprisingly the thoughts crossed the mind of scientists too. And they have an answer!

New research in the field of the Covid-19 vaccine and its effectiveness has revealed that mixing two vaccines can trigger a potent immune response. The studies conducted in the UK also suggest that combining two vaccines might also outperform the results from two shots of the same vaccine. Additionally, German studies on the same lines suggested a similar reaction.

Following the results from the study, immunologist Leif Erik Sander at Charité University Hospital in Berlin also expressed that, with the results of the studies, people can "feel a bit more comfortable" about mixing vaccines.

The results have also given researchers confidence that if other vaccines — apart from Oxford–AstraZeneca jab and the Pfizer–BioNTech vaccine — were mixed, the response would be similar.

What prompted the study?

The prompt for the study goes back to the time when in some instances Oxford–AstraZeneca vaccine caused a blood-clotting condition known as thrombosis with thrombocytopaenia. Following this, some European countries decided to halt the administration of the said vaccine. This halt left people who had already received the first dose of the vaccine partially vaccinated.

With uncertainty around when the Oxford–AstraZeneca vaccine would become available, there was a need to estimate the reaction if a candidate received two different vaccines. Thus further prompting the need for a trial.

Following this, in May, researchers at Madrid announced the results from a trial conducted to understand the reaction from combining two vaccines — CombiVacS trial.

According to the study, the people who received Pfizer 8-12 weeks after receiving a dose of Oxford-AstraZeneca displayed stronger immune response. The authors of the study found that people who received the combination of vaccines produced 37 times more Covid-19 neutralising antibodies. They also found that there were four times more SARS-CoV-2-specific immune cells, called T cells. These statistics are in comparison to the people who had taken one dose of Oxford-AstraZeneca.

For the purpose of the study, the researchers looked at 340 healthcare workers who had either received two doses of Pfizer–BioNTech vaccine, or an initial shot of the Oxford–AstraZeneca vaccine followed by a dose of Pfizer–BioNTech. Both the vaccines triggered an immune response that neutralised antibodies and formed T cells.

Another study conducted by researchers at Saarland University in Homburg, Germany, also had similar results. They found that the mixed vaccines induced an immune response that was better than a single shot.

However, one shortcoming of the trial is that the study group was comparatively small to ascertain the efficacy of the mixed vaccine shots. Martina Sester, an immunologist who led the Saarland study said, "As long as you don’t have any long-term or any follow-up studies with efficacy calculations, it’s hard to say."

The concerns

When it comes to mixing vaccines, the biggest challenge is that of the unknown. There is much more clarity required pertaining to which vaccines can or cannot be mixed. Or the order in which these should be administered.

Other complexities have also been highlighted by International bodies such as Coalition for Epidemic Preparedness Innovations. These bodies are majorly responsible for looking into the mixing and matching of the Covid-19 vaccine.

The three highlighted concerns include — the shelf life of the vaccine, the shipment process, and storage conditions. These conditions can make a difference as to how a person taking a mixed vaccine might react. They also mention that some vaccines may also have side effects on people with certain conditions.

However, to date, there has been no issue (theoretically) that could make the mixing of vaccines a safety threat. Talking to Indian Express, Dr Kang says, "Our immune systems are capable of handling a lot — we are seeing an increase in minor side-effects with mixing, but do not expect major side effects."