Covid-19 vaccine mix-and-match: Safety, efficacy and protection against emerging variants.

The increasing demand and dwindling supply of Covid-19 vaccines has led to some countries (e.g. China and Germany) resulting in a mix-and-match approach. Side effects from vaccines such as AstraZeneca, that include blood clots, have led to public fears and those who have had the first shot are unwilling to get the second from the same manufacturer. The mRNA vaccines (eg. Pfizer-BioNTech) require special storage and transportation facilities, which remain unavailable in most developing countries. Additionally, the demand for Covid-19 vaccines is also increasing as awareness increases and more people are willing to get vaccinated.

Mixing of the available Covid-19 vaccines is being adopted to match the demand with the supply. Regulation bodies including the Center for Disease Control and Prevention (CDC) and the UK Public Health England have recommended mixing of doses where there is shortage of the first dose during the second booster. Vaccine combinations mount different pharmacokinetic activities against viruses. Different vaccines have different potency and their efficacy against viruses increase when they are combined. A combination of vaccines could also enhance immune system response hence combating the emerging variants that could be resistant to the initial dose. In the case of Sputnik V, the prime and the subsequent booster dose use different methods to deliver the viral genetic material to the immune system eliciting the highest immune response compared to the rest of the vaccines. Similar to Sputnik V, mixing of other vaccines could boost the overall efficacy against SARS-CoV-2 and its variants.

Adverse events in specific age groups resulting from first dose of Covid-19 vaccines, has led to their contraindication. This necessitates the use of a different vaccine for the booster shot. However, preliminary data from the world first study on Covid-19 vaccine mixing (Com-COV) has shown that mixing AstraZeneca and Pfizer vaccines, the former being the most common in developing countries, shows increased frequency of mild and moderate reactions such as fever, aches and chills compared to standard dosing approaches.

Adoption of a mix-and-match approach is considered practical despite the limited data, especially data on vaccines developed through different platforms such as mRNA and adenovirus-based vaccines. Previous research experience with HIV among other viral diseases have paved the way for vaccine mixing. Although Covid-19 vaccine mixing studies on induced immune response, safety profile and the duration of active immunity are ongoing, the benefits outweigh the risks and the goal is to fully vaccinate the majority within the shortest time possible.