Even as the academic research is underway to find the efficacy of the two vaccines being administered against COVID-19, the inactivated virus vaccine mechanism is supposed to be ideal to generate required amount of immunity as the efficacy period of both the vaccines is stated to be about 9-12 months. 

Senior virologist Dr Jacob John says that the use of inactivated virus vaccines after two doses of vectored virus vaccines will serve as a booster vaccine and enhance the antibody response against COVID-19. "The response of a third dose of vectored vaccine might not be as effective as an inactivated virus vaccine because antibodies generated after first two doses are likely to inactivate the virus as their booster response starts. Thus, those taking vectored vaccines, inactivated virus vaccines will help in enhancing the immunity," says Dr Jacob John. 

Meanwhile, the researchers are studying the impact of third dose of inactivated virus in healthy individuals after six months of complete vaccination. Dr Sathyajit Mohapatra, lead researcher of Covaxin trials in SRM Medical College and Research Centre, said that the efficacy of the booster doses of inactivated virus in case of Covaxin is being studied to keep a check on the adaptations to be made as more number of variants are surfacing as the immune response of the Covaxin differs in case of Delta variant and Delta Plus variant. "The adaptations in terms of other variants have to be studied and mixing of the vaccines might be beneficial, as foreign studies suggest. However, the mixing of vaccines also has its concerns pand the side effects in those with comorbidities will be crucial," he said. 

With the ongoing shortage of vaccines and unavailability, the mixing of vaccines for third dose will only lead to rush and confusion, says public health expert Dr K Kolandaisamy. He added that as the nature of the virus changes over time and variants develop, it is necessary to modify the vaccines for third dose.