How many need to be vaccinated to break Covid-19 transmission chain

The Covid-19 vaccine race is unprecedented in the history of mankind. With the United Kingdom becoming the world’s first to approve Covid-19 vaccine, the race has become even fiercer. The crucial question is: how many should be vaccinated to slow down the pandemic?

Like every other country, India is also drawing its vaccine distribution strategy. India’s apex health research body – the Indian Council of Medical Research (ICMR) – recently said it may not be necessary to vaccinate the entire population with Covid-19 vaccine as inoculating a critical mass of people might be enough to break the chain of viral transmission.

Pause. Imagine a forest fire. It may be impossible to put out every tree that’s already on fire, but it may be possible to prevent the blaze from spreading to larger areas. We don’t need to extinguish every tree that is ablaze to prevent the fire from spreading.

Similarly, a vaccination campaign may miss many individuals, but vaccinating a certain threshold will reduce the spread of the virus in the larger community. Each individual who is vaccinated gets individual protection but all the vaccinated individuals together provide a population-level protection effect called herd immunity.

When a large enough proportion of a population has been vaccinated and is immune to an infection, their collective immunity protects those members of the population who are not immune. This is known as herd immunity or population immunity and it is used in the context of vaccination.

Here’s a small story to illustrate the point

Meet the famous trio from the 2009 Bollywood-hit 3 Idiots set in the time of Covid. Rancho ventures out quite often and contracts coronavirus. He doesn’t wear a mask when he meets Raju who also gets infected. Farhan doesn’t keep distance from Raju and becomes the third link in the transmission chain.

Any three of them may start an outbreak in the Imperial College of Engineering. Most students may recover but if the college director – nicknamed ‘Virus’ in the film – gets Covid-19, he may not.

If we only had enough vaccine for one person, vaccinating Rancho will provide protection to him but may stop the transmission chain. If we vaccinate many students and teachers, the virus may not be able to spread in the college even if a few students did not get the vaccine. If there are enough people who have been vaccinated, the chain of transmission is interrupted by immune persons and the virus can no longer spread.

Spread of Coronavirus

R0 (pronounced R naught) represents the number of infections that a single infected person is expected to cause in the population at the beginning of an outbreak and describes the overall transmissibility of an infection in a community. If the R0 is greater than 1, the outbreak will continue to grow. Conversely, if R0 is less than 1, the spread of infection will decrease and the outbreak will fizzle out. With time many people may become immune after infection or vaccination.

Re, the effective reproductive number, represents the number of people in a population who can be infected by an infected individual under specific circumstances; it better captures changes in transmission of infection as some individuals become immune following infection or vaccination, as people change their behaviour and as control measures like lockdowns are enacted.

The critical threshold at which transmission of the virus is interrupted is called the herd immunity threshold (HIT). The HIT for SARS-CoV-2 is estimated to be around 60 per cent but that doesn’t mean the virus will stop spreading at 60 per cent.

Imagine you’re driving a car, and you brake suddenly, the car slows down but it doesn’t stop instantly. Similarly, because the virus is spreading so quickly, it will slow down at the HIT but may overshoot to 70 or 80 per cent.

Reaching a herd immunity threshold doesn’t mean the pandemic will end, it may slow down under specific conditions but as we have learnt, the infection can still spread in specific settings.

Even if 60 per cent of the population is immune due to vaccination or infection and the Re has dropped below 1, some susceptible people will come into contact with infectious people in settings like bars, indoor dining, movie theatres and subway trains.

A Re less than 1 during a lockdown may quickly increase as restrictions are lifted. This adds some complexity to calculating the vaccine coverage required to stop the pandemic.

Maths of vaccine coverage

Not everyone will get vaccinated, vaccines are not 100 per cent effective, and the number of people who have already been infected and recovered is ever increasing. Therefore, there is some complex mathematics needed to understand vaccine coverage. To extinguish the ongoing pandemic without any other measures like physical distancing, the vaccine needs an efficacy of at least 80 per cent with 75 per cent vaccine coverage but with 60 per cent efficacy, it might need 100 per cent coverage.

As the population infected by the virus increases, the ability of the vaccine to impact the pandemic and reduce the size of the pandemic’s peak decreases. Once 30 per cent of the population has been infected, we will almost be at the peak of the pandemic, and the vaccine’s impact on the pandemic will be reduced.

We need an effective vaccine and need to vaccinate a larger population but we also need to do it very quickly to have maximum impact. If the vaccine is combined with other measures like mass testing, universal masking and physical distancing, we may need to vaccinate even fewer people. With a layered approach, no single measure needs to be 100 per cent effective or have 100 per cent coverage.

A layered approach uses multiple interventions together such as a vaccine, mass testing, masking and social distancing to prevent the infection. Taken together, these measures may just end the pandemic even without 100 per cent vaccine coverage.

On December 2, just a year after the first recorded case of Covid-19, the UK approved the Pfizer-BioNTech mRNA-based Covid-19 vaccine. Despite the remarkable scientific breakthroughs that have brought us several vaccine contenders, some experts have correctly pointed out that innovation is nothing without distribution. That begs the question of how much vaccine distribution is required to end the pandemic? Well, it depends on a few factors.

Infographic: India Today DIU

The government plans to vaccinate around 25-30 crore priority population, including healthcare and frontline workers, elderly population and those with co-morbidities in around six months once the vaccine is available for use.

Different vaccines, different immunity

Not every vaccine produces the same kind of immunity. Some vaccines may prevent someone from getting very sick or dying but might not prevent mild or asymptomatic infections. The vaccinated person will have some protection but may still infect others. Other vaccines may prevent infection and death in young people but may not protect the elderly from the risk of death.

Some vaccines may prevent shedding of the virus completely or partially and thereby reduce infectiousness.

Vaccines may also have different efficacy in an Indian population compared to elsewhere. While we can be cautiously optimistic that the Covid-19 vaccines will prevent infection by SARS-CoV-2, deaths and shedding of the virus, this is far from proven. The number of people we need to vaccinate and who we chose to vaccinate will also depend on the kind of immunity the vaccine provides.

The end is nigh, be cautious!

While it is difficult to answer the question of vaccine coverage required to stop the pandemic with certainty, the answer to the question of how many people we should vaccinate is simple: as many as possible and as quickly as possible. We have built the impossible vaccine in an impossible timeline, now we need to distribute it to an impossibly large population.

One of the world’s leading infectious disease experts, Dr Anthony Fauci, said, “Don’t take the vaccines as an invitation to throw caution to the wind. It’s kind of like the last soldier to get killed in a war when the war is going to be over soon. You don’t want to be that person.”

(A Covid-19 clinical researcher, Dr Swapneil Parikh is author of The Coronavirus: What You Need to Know About the Global Pandemic)

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