In the beginning of the pandemic, when vaccines were still undergoing clinical trials, the goal was to quickly reach herd immunity through vaccination and natural infection. Herd immunity — where a large percentage of the population is infected or vaccinated so that virus spread in the population is significantly slowed or stopped — was seen as an endgame of the pandemic.
There have been instances in some countries where the virus spread has largely been under some kind of control at least for some duration. However, the emergence of highly contagious new variants, breakthrough infections even among the fully vaccinated and such people spreading the virus to others, and reinfections in the unvaccinated have made the race to reach the magical herd immunity threshold to successfully break the transmission chain and return to normal times look like chasing a mirage.
No magic number
“If herd immunity is viewed as a magic number where, if we achieve X percent infection or vaccination, life will get back to normal then that is not going to happen in near term. In other words, attaining high levels of population protection to slow virus spread is an end goal, but is not the endgame,” Dr. Gagandeep Kang, Professor of Microbiology at CMC Vellore says in an email to The Hindu.
Immunologist Dr. Satjajit Rath, formerly with the National Institute of Immunology, is even more blunt in dismissing the notion of achieving herd immunity to control the SARS-CoV-2 virus spread. “Herd immunity is not, in fact, it cannot be, a prospective end goal for a public health policy, especially one aimed at a globally spreading new infection. We have no idea what percentage of population coverage will end up being effective [for] herd immunity,” he says in an email. “It is quite likely that any state of such ‘herd immunity’ in a given community will not be stable — people move, thereby changing the demography of the ‘herd’; variants emerge, thus making the ‘immunity’ less effective, and all of these changes can well lead to loss of herd immunity.”
Dr. Rath adds: “The idea that we can aim for a stable situation of herd immunity and then just stop worrying about the pandemic has always been more wishful than real.”
Role of variants
If the Alpha (B.1.1.7) variant was 60% more transmissive than the original strain, the Delta variant is even more contagious thus requiring an even higher herd immunity threshold to break the transmission chain. It is estimated that the Delta variant may have a basic reproductive number of between 6.5 and 8, which means that herd immunity will be 85% or higher.
The next layer of complexity is seen when even fully vaccinated people get infected by the Delta variant, and such people shed virus at levels that resemble previously uninfected individuals and thus infect others. “But the duration of shedding may be shorter, which means that the overall ability of the virus to spread in previously infected or vaccinated individuals will be lower,” Dr. Kang says.
Even if the duration of virus shedding in the vaccinated is short, it makes it harder to break the transmission chain even in a population that is highly vaccinated. Thus, even the theoretical possibility of protecting through herd immunity those individuals who cannot take a vaccine gets diminished. “There is no question of ending transmission until we end infections,” she adds.
According to her, the data from Israel seems to indicate that there is strong protection from transmission with the mRNA vaccines shortly after vaccination, and this then declines with time. But she says that these are early data, and there is a lot to learn about the performance of other vaccines and over time.
Help from vaccines
“There are many more vaccines to come, and it is feasible that the next wave of vaccines or combinations of vaccines could help drive down transmission through lower levels of replication and better protection from infection,” says Dr. Kang.
“Vaccines are needed for almost everyone. Expecting the benefit to pass on to the unvaccinated does not seem to be possible with this virus,” explains epidemiologist Dr. Giridhara Babu from the Public Health Foundation of India, Bengaluru. Dr Rath adds that except for very rare cases of someone who already has, for example, a major life-threatening allergy-related illness, there are really no individuals who cannot take a vaccine; there are only those who do not want to take a vaccine.
Small surveys
The over 63% average seroprevalence in the country combined with increased vaccination coverage can help reduce the speed of virus spread. But more importantly, natural infection and vaccination can decrease the risk of severe disease and death even when reinfected. But the over 63% seroprevalence is only an average and has been arrived at based on a survey of less than 30,000 people in less than 10% of India's districts. “It does not mean that two-thirds of every neighbourhood is seropositive; the reality is much more of a patchwork quilt, making any easy predictions misleading,” Dr. Rath clarifies.
This makes quickly vaccinating a large percentage of people very important. Unfortunately, the pace of vaccination is slow in India — less than 33% are vaccinated with one dose and less than 9.5% fully vaccinated — and vaccine shortages in many States have become a norm. This makes reaching the imaginary herd immunity threshold anytime soon a big challenge.
Even among the vaccinated, inequity in terms of geography and socio-economic strata is rampant. “This inequity means that the reality will be well-vaccinated and poorly-vaccinated communities living cheek-by-jowl, making any idea of stable herd immunity even less likely,” Dr. Rath notes.
Lasting immunity
Finally, by the time the entire country reaches very high seropositivity either through vaccination or natural infection to achieve herd immunity, it remains unclear if protection would last in those who have been infected/vaccinated early on. It is likely that protection from severe disease and mortality might last for several years. However, there is no data at the moment.
While Dr. Babu feels that the endgame might be a situation where we would have to live with the virus with some level of endemicity but with reduced hospitalisation and lower mortality, Dr. Kang says it is too early to give up on herd immunity.
“There are viruses that are much more infectious than SARS-CoV-2, such as measles. We have achieved reasonable control of measles and are striving to do better. We have not hit the herd immunity threshold in many parts of the world, and yet measles is no longer the major global killer it once was,” she says.
