The Second Dose

With the U.K. regulator granting an emergency use authorization for AstraZeneca’s COVID vaccine on December 30, three vaccines have now been greenlighted for use even before full approval is granted. Unlike the mRNA vaccine platform of the Pfizer and Modern vaccines, AstraZeneca uses a weakened version of a common cold virus (adenovirus) that carries the genetic material of the spike protein to elicit an immune response. Requiring only 2°­8°C for transport and storage, it can be administered in existing health­care settings, allowing rapid deployment.


Together with the cost, which is much cheaper than the mRNA vaccines, this is particularly critical to help end the pandemic in the global south. The emergency use approval makes it easier for similar assent to the vaccine tested and manufactured in India. However, the European Medicines Agency, on the same day, clearly said that “additional scientific information on issues related to quality, safety, and efficacy of the vaccine is deemed necessary” to give a similar nod. While published results show only 62% efficacy when two full-strength doses are administered four weeks apart, the U.K regulator has relied on unpublished data showing efficacy increasing to 73%, 22 days after the first dose when the second jab was delayed.


Most of the participants in the phase­3 trial in the U.K. including in the primary analysis received the second dose more than nine weeks after the first. Based on this information, the regulator has permitted the second dose to be administered 4­12 weeks after the first jab. The intent is to maximize the vaccine’s benefits by stretching the time between two doses to allow more people to get vaccinated, an untested strategy to rapidly but partially protect more individuals who are at risk. With the virus running amok, the U.K. has revised its policy to delay the second dose of the Pfizer vaccine too despite no trial data to support this; the second Pfizer jab is originally scheduled three weeks after the first.


While the regulator says there is 89% short term protection from day 14 after the first dose, Pfizer insists that there is “no data to demonstrate that protection after the first dose is sustained beyond 21 days”. Two doses of both vaccines complete the course and prolong the duration of protection. If the pandemic forced scientists, companies, and regulators to quickly make available safe and efficacious COVID­19 vaccines, delaying the second dose to maximize the benefits is altogether a new strategy. The untested approach might probably be appropriate for the U.K., which is facing the brunt of the virus, as a desperate measure. Since delaying the second dose appears to be increasing the efficacy, and as the U.K. has approved a delayed second dosage, India should carry out large trials to test this.



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