The world desperately needed a vaccine to reduce the misery caused by SARS-CoV-2. Outstanding work done by scientists, vaccine industry, funding agencies and governments has resulted in a clutch of vaccines being made available within a year of the declaration of the pandemic.
The general expectation is that when the vaccine against Covid-19 is rolled out in the population it would mark the end of the pandemic, the virus would go away and people can start pre-Covid ‘normal’ life again. Many are counting on the vaccine to build enough herd immunity, resulting in slowing down of the transmission and finally stopping it.
In general, a vaccine can curtail the likelihood of a person getting infected, and/or reduce the chances of development of symptomatic disease, and/or diminish chances of progression into severe disease, and/or curtail the probability of transmitting the infection to others. A vaccine which acts on one or more of these stages can contribute to disease control.
A quick look at how some of the vaccines have worked gives valuable insights. One of the most impactful vaccines has been the one for smallpox. It produced sterilising immunity and left almost no chance of getting infection and spreading it to others. It helped the world to eradicate the disease. Human papilloma virus vaccine also generates very effective sterilising immunity.
But not all vaccines are able to offer this. Rotavirus vaccines can decrease viral replication, lower the viral load, which decreases transmission, but mild disease continues to occur. Neither does the measles vaccine. Pertussis vaccine protects from illness but an infected child is able to infect others. Even without sterilising immunity these vaccines have played a very decisive role in disease control.
For the vaccines against SARS-CoV-2 one of the end points used in almost all the Phase 3 vaccine trials is to reduce symptomatic laboratory confirmed Covid-19 disease. The term efficacy relates to the ability of the candidate vaccine to decrease the development of symptomatic laboratory confirmed disease compared to the group which did not receive it – the placebo group.
A vaccine efficacy of 90% (like those of Pfizer-BioNTech, Moderna, Russian Sputnik V) means that these vaccines are capable of effecting a 90% reduction in cases of the disease in the vaccinated group as compared to the placebo in all age groups including those beyond 65 years. Similarly, a 62% efficacy in those below 65 years (like that of AstraZeneca’s Covishield, when used in two standard doses – as are given in India) translates into a 62% reduction of the disease.
Roughly 2 in 5 people who receive the vaccine may still catch the disease. The efficacy in older populations is uncertain. However, interim analysis of limited data indicates that in those above 65 years it is about 52%.
The currently available SARS-CoV-2 vaccines reduce development of symptomatic laboratory confirmed Covid-19 and also lessen the chances of developing severe disease. Such vaccines would be very helpful in reducing the hospital admissions. The stress on the healthcare system would be reduced.
Any vaccine that reduces the incidence of symptomatic infections will also lessen the transmission of the virus. But people with asymptomatic and symptomatic infections can still spread the virus, albeit less effectively. As of now it is uncertain to what level they also block transmission.
Another issue that people keep referring to is development of herd immunity through vaccination. Herd immunity is a function of vaccine efficacy and the percentage of population immunised.
R0, the basic reproduction number, for SARS-CoV-2 is 2.5 to 3.5 – this is the number of persons an average infected individual is able to infect. The aim of developing herd immunity is to reduce R0 to less than 1, so that it does not remain a public health concern. There is a fair amount of consensus that we need to develop 60-70% herd immunity for this to happen.
Simulation exercises have revealed that to extinguish an ongoing epidemic, a vaccine which has an efficacy of 60% would require 100% of its population to be immunised. The coverage level drops to 60% if a vaccine has 100% efficacy. For a vaccine which has less than 60% efficacy, herd immunity is unachievable and epidemic control is substantially poor. Given the proportions of population needed to be vaccinated with vaccines of high efficacy, development of herd immunity may be difficult to achieve in India.
But even if the Covid-19 vaccine is only able to convert a SARS-CoV-2 infection into an upper respiratory tract illness, it would be a lot better and manageable. Hospital admissions and deaths are likely to decrease.
What the scientists and the government have done is extraordinary, it is now our turn to reciprocate. We should derive full benefit from the opportunity and get vaccinated in large numbers. Any protection is better than no protection.
The virus is likely to stay with us, and much like the pandemic strain of influenza virus it would circulate in the community causing outbreaks whenever the conditions are favourable. We cannot afford to discard the face mask or abandon the precautions of keeping safe distance and hand hygiene yet.
Views expressed above are the author’s own.