Nearly three months after Bharat Biotech’s intranasal COVID-19 vaccine was granted emergency use approval for primary vaccination of adults, the vaccine has now been given emergency use approval as a heterologous booster in adults over 18 years. Unlike Covaxin, an inactivated vaccine, the intranasal vaccine uses a viral vector platform that utilises a recombinant adenovirus as a vector. As a heterologous booster, the intranasal vaccine was tested on around 875 participants, immunised earlier with two doses of either Covaxin or Covishield. Among the advantages intranasal vaccines have over vaccines is that they can potentially prevent SARS-CoV-2 infection at the point of viral entry in the respiratory tract. They are also far easier to administer. As in the case of the intranasal vaccine for primary vaccination, the booster dose too was tested only for safety and immunogenicity, but not efficacy. Unlike the vaccines tested early in the pandemic, evaluating the efficacy of a new vaccine for primary vaccination and heterologous booster at this stage of the pandemic might be very challenging as a vast population has been already vaccinated and/or naturally infected, especially with the Omicron variant.
However, even higher immunogenicity need not necessarily translate into higher efficacy, particularly the ability of the vaccine to prevent infection. The SARS-CoV-2 virus has now acquired an even greater ability to evade existing immunity and cause an infection. Since the intranasal vaccine does not have the virus spike protein with newer mutations, it is important to study within a defined time period the effectiveness of the intranasal vaccine to prevent infection and severe disease and death when used as the primary vaccination and as a booster dose. Alternatively, the vaccine’s effectiveness can be tested in human challenge studies. Studying this becomes important as the results of an intranasal vaccine in the phase-1 trial using the Oxford vaccine — an adenovirus-vector vaccine — originally developed for intramuscular administration, elicited antigen-specific mucosal antibody responses only in a “minority” of participants, and the antibody levels were far lower than from a natural infection. The Oxford vaccine, as an intranasal vaccine, did not elicit “consistent mucosal antibody response nor a strong systemic response”. The demand for vaccines for primary vaccination has been reducing since the third wave peaked in end-January while those for boosters has been low even when available for free for 75 days beginning mid-July. Yet, the development of an intranasal vaccine, a new vaccine platform for India, is welcome, and having a safe and effective vaccine that can prevent infection remains a high priority.