Earlier this week, the Indian Council of Medical Research (ICMR) stated that, while efforts are being made to generate a COVID-19 vaccine with close to 100 per cent efficacy, the first vaccine that gains approval may have an effectiveness anywhere between 50 and 100 per cent. ‘100 per cent efficacy is not achieved in vaccines for respiratory viruses,’ ICMR Director General Dr Balram Bhargava added, citing the World Health Organisation’s guidelines. 

With cases continuing to mount, and India struggling to suppress its daily COVID-19 tally, the nation is effectively pinning its hopes on the development of a vaccine to be distributed as swiftly and widely as possible. But will a vaccine with a lower effectiveness be able to end the outbreak in India? 

The answer depends on a variety of factors. In a study published in July in the American Journal of Preventive Medicine, researchers created a computer simulation of the entire US population to determine how effective a vaccine will need to be before enough herd immunity can be achieved. They found that a coronavirus vaccine will need to have an effectiveness of between 70 and 80 per cent such that social distancing and mask-wearing protocols can be dropped. 

Say a vaccine’s efficacy is 70 per cent. This means that, on average, out of a cohort of 100 people, 70 will develop immunity to the virus. However, efficacy and effectiveness are not the same thing, with the latter used to describe how much protection a vaccine will provide under ‘real world circumstances.’ As such, a vaccine’s effectiveness is always likely to be lower than its efficacy. 

What does a vaccine’s effectiveness depend on?

In a country with a population as large and demographically diverse as India, it is unrealistic to expect that the entire citizenry can be administered a vaccine within a handful of months. The effectiveness of a vaccine will primarily hinge on how widespread the disease already is, how quickly it is spreading (its reproduction rate or R0), and how large a proportion of the population receives it. 

For instance, if a vaccine was discovered and administered right at the start of the outbreak when the population infected was close to 0 per cent, the US researchers found that vaccine efficacy would need to be, at least, 60 per cent to completely end the outbreak, assuming that the entire population was vaccinated. 

However, if just 75 per cent of the population was vaccinated, a vaccine’s efficacy would have to be closer to 70 per cent. It follows then that the fewer members of a population that have been vaccinated, the more effective a vaccine would need to be. 

How contagious a virus is will also have a huge role to play in how effective a vaccine is. The more contagious it is, the more effective a vaccine will need to be. Simply put, if the virus is spreading at breakneck speeds, the rate of infection will outstrip the rate of immunisation, thereby limiting the overall protection a vaccine offers. The researchers found that if 5 per cent of a population was infected with a virus that had an R0 of 2.5, even in the best case scenario, the peak can only be reduced by 85 per cent. 

This isn’t to say that a vaccine with an effectiveness of 50 per cent is of no value at all. Ultimately, any measure capable of reducing the infection rate of the virus will better enable the country to return to a state closer to normalcy, with businesses allowed to function and people allowed to roam freely. 

It is overly optimistic to think that the first vaccine produced in India will immediately put an end to the outbreak. However, the ICMR’s announcement remains encouraging, and will, without doubt, set India on a path towards achieving complete herd immunity much quicker, and in a much safer manner than it is doing right now.