It is heartening that the Pune-based Serum Institute of India has received the country’s top drug regulator’s approval to conduct late-stage human trials for the Oxford-AstraZeneca COVID-19 vaccine candidate. The vaccine has shown encouraging results in early trials. Its mild side effects augur a safe vaccine. But scientists have rightly advised the tempering of hope with caution: Large-scale trials are required to ascertain if the vaccine actually protects against the pathogen.
The immune system has two ways to defend the body against pathogens like the coronavirus — by producing antibodies that disable an infectious agent and by manufacturing T cells that have a long memory of such agents. Early research has suggested that antibodies fade away within months of contracting the coronavirus. However, a robust T cell response can prevent the infection from recurring. The Oxford-AstraZeneca vaccine is designed to trigger both kinds of reactions. But we do not know how protective the memory in the T cells, induced by the vaccine, will be against the coronavirus. Moreover, the trials were conducted on volunteers aged 18-55. The efficacy of the vaccine in older, and more vulnerable, people is still not known.
According to the WHO, more than 150 vaccine candidates are undergoing trials in different parts of the world. Three of them — the Oxford vaccine, the vaccine being developed by the pharma major Moderna in the US and the candidate of the Chinese firm, Sinovac Biotech — have reached the third stage. These tests are usually the most extensive before a vaccine is given regulatory approval. The Oxford vaccine, for instance, was administered to a little more than 1,000 people in the initial trials. It will reportedly be tested on more than 30,000 people in different parts of the world — 1,600 of them in India — in the final phase. The US FDA lists more than 20 cases where the results of the Phase 3 trials diverged from the initial tests. Moreover, though vaccine research has made enormous progress from the 1950s, when thousands of children developed a milder form of polio after being inoculated, there are reasons to be cautious even after a COVID vaccine enters the market. In 1999, a rotavirus vaccine had to be recalled after it caused adverse reactions. It would also be worth remembering that flu viruses mutate. We are still some way away from victory against the coronavirus.