Setting vaccine effectiveness threshold at 50% is a compulsion, as CDSCO has, but need to consider pitfalls too
And, even if eradication is not a feasible short-run goal, trying to retard a disease like Covid-19 meaningfully will require a large chunk of the population developing immunity through the potential vaccine.
India’s drug quality regulator, the Central Drugs Standard Control Organisation (CDSCO), has said in its draft regulatory guidelines for vaccine development that a Covid-19 vaccine candidate that demonstrates 50% effectiveness in placebo-controlled trials can be eligible for a national roll-out. Globally, the leadership of individual nations’ pandemic-response seems to believe the chances of a highly-effective vaccine are slim—Anthony Fauci of the National Institute of Allergy and Infectious Diseases, the US, has said the chances of “75% or more” effectiveness are “not great”, while Balram Bhargava, director-general of the Indian Council of Medical Research has said we may get “anywhere between 50% to 100%”. Indeed, with the WHO calling for a 50% threshold for vaccine efficacy (with the minimum duration of protection being six months), many national regulators, including the US’s Food and Drugs Administration, have formally adopted this threshold, pleading “special circumstances”. To be sure, it is expected that immune-system response will be impaired in specific high-risk groups for Covid-19, such as the elderly, those with co-morbidities like diabetes and hypertension, etc. However, to start with a low benchmark is a fraught proposition—especially when viewed against the fast-tracking of approval that has marked global Covid-19 vaccine development efforts, which is likely to have sidestepped some of the scientific rigour needed for testing of vaccines.
While it is broadly held that a vaccine must have an effectiveness of above 70% to halt the spread of its target pathogen, the fact is, even in a good year, the flu shot effectiveness (at roughly 60%) mirrors the threshold that nations are now considering for Covid-19. Yet, it continues to be strongly recommended by doctors in many jurisdictions. Some experts argue that even if a vaccine fails in half the vaccine subjects, it may still end up having trained the subjects’ immune system to a degree where the severity of the disease, when they contract it, is lower than expected otherwise—leading to lower demand for hospitalisation/intensive care, and a lower number of deaths. This has been noticed for the flu shot, but determining this for Covid-19 is going to take deeper research. Given some of the high-risk groups may have impaired immune systems, this doesn’t look to be a certainty. Contrast this with the fact that most childhood vaccines deliver an effectiveness of 85-95%, a crucial factor behind the eradication of some diseases. And, even if eradication is not a feasible short-run goal, trying to retard a disease like Covid-19 meaningfully will require a large chunk of the population developing immunity through the potential vaccine. Low effectiveness means the vaccination coverage has to be that much larger to get to the required strength of immune people. Vaccine-makers have already voiced concerns that the pipeline may not be large enough in the initial years, a problem compounded when multi-dose vaccines come into play; the follow-up shot has to come within a relatively short duration of the first dose.
Apart from these concerns, a vaccine that doesn’t do too well—50% effectiveness is unlikely to enthral people—could spur vaccine hesitancy, or worse, denial; polls in the US have already shown significant levels of reluctance amongst people. But, the worst scenario that could emerge is the nudge-effect a vaccine—even when the subjects know that it is not super-effective—could have on the subjects. Imagine still-susceptible vaccine recipients dropping their guard on the use of masks, hand-washing, distancing, etc. If there is no choice but to roll out relatively low-effectiveness vaccines, the government will have to ensure that vaccine subjects are tracked, tested periodically for development of immunity (the metrics of this has to be also developed in consensus with other jurisdictions), and receive counselling on risk-appropriate behaviour. The US already proposes to do this for a year from the inoculation date, though it is not clear if this will involve sero-tests of each subject to monitor the immunogenic effect of the vaccine. India must put in place a system to do exactly this.