Herd immunity or herd protection is an epidemiological concept that defines a situation when a high percentage of the population is vaccinated to make it difficult for an infectious disease to spread.
For example, let’s consider a disease like measles. Measles always spread like wildfire, because it has the highest basic reproduction rate or an R0 of 12 to 18. In simple terms, R0 or R naught is a way to calculate how many people, on average, are likely to get infected through contact with one infected person. In the case of measles, one infected person can pass the disease on to 12-18 people.
A highly contagious disease like measles can quickly disappear if an infected individual is surrounded by people who are vaccinated against it. For herd immunity to work, at least 19 out of the 20 need to be vaccinated against measles to protect the vulnerable population.
In a nutshell, herd immunity only works when a major percentage of the population is vaccinated against the disease.
For measles, 93% to 95% of the population must be vaccinated to achieve herd immunity. Thankfully, COVID-19 is far less infectious than measles. Even though the R0 for COVID-19 is roughly 3, the proportion of persons who need to be infected to attain herd immunity is around 70% of the entire population.
Consider infecting 70% of the population with COVID-19, this by no means can be labeled as a preventive strategy. 70% is a huge number that doesn’t just include the young people, but also the geriatric population, newborns, and even those who are immunocompromised.
Furthermore, with the case fatality rate (CFR) of 3.4% in India, if 70% of our population is infected with COVID-19, then roughly, 3 million people in our country could die. Also, the case fatality rate for herd immunity will always be on a higher side and so, the outcome will be catastrophic for any country.
Comparably, COVID-19 has been far more deadly in Sweden- a country imposing minimum restrictions and advocating herd immunity- as compared to its neighboring countries. With just 16000 cases as of 23rd April 2020, Sweden has already hit a toll of 1,937 deaths, which makes its CFR roughly 12%, bringing it closer to countries worst hit by COVID-19 pandemic like Italy and France.
Hence, those who are supporting the “Swedish model” or are tempted by “herd immunity” must understand that this strategy brings with it a defining feature of a higher death toll.
Not just that, there are many repercussions of this devastating scenario. Considering a need for around 10% of the infections to be hospitalized, such an enormous number of hospital admissions will have a huge impact on our health care system.
It is hard to predict the outcome of a pandemic. But amid this uncertainty, we are sure of one thing and that is to limit the spread of this deadly virus and herd immunity without a vaccine will fall flat to solve that purpose.
The unfortunate reality is, until we have a vaccine, talking of herd immunity is similar to fueling our pandemic woes. Even in the case of past pandemics like the Spanish flu of 1918, with a similar R0 as COVID-19, 70% of the population did not get the flu. The Spanish flu came and went away in natural cycles. Hence, hoping that herd immunity will save us from COVID doesn’t sound convincing.
At present, the only preventive strategy for COVID-19 seems to be practicing social distancing and avoiding people who are sick.