What is herd immunity and can it help stop COVID-19?

Herd immunity as a concept emerged from veterinarians more than a century ago in the United States, where they were studying various epidemics among livestock across the country. The phrase "herd immunity" first appeared in the Journal of the American Veterinary Medical Association in 1918, where two authors of a paper explained that it could be developed "by retaining the immune cows, raising the calves and avoiding the introduction of foreign cattle". The authors were discussing the epidemic of spontaneous miscarriage, also known as "contagious abortion", which was plaguing the country's farms, prompting them to sell or kill affected cattle.

Herd immunity is an epidemiological concept that means a large majority of the population is immune to a contagious infection through vaccination or by developing immunity through an infection that occurred earlier. This allows those who weren't vaccinated to become naturally immune through the robust immune system of those around them.

Through this method, a large population can gain immunity over a sustained period of time. It has been a successful method in the fight against diseases such as diphtheria and smallpox in earlier times. It is also how measles, another infectious disease with a reproductive rate far higher than the coronavirus infection, was also controlled. 

Our body's immune system develops antibodies to fight off any disease - these antibodies are specific to whichever virus or bacteria they are fighting. In the case of herd immunity, a person who may not be immune to a virus but is part of a community of people who are all immune doesn't catch the infection. The reason, as one scientist put it very simply, there's no one to catch the infection from!

Early researchers into the concept of herd immunity, however, had different definitions to its modern version as our understanding of the concept has evolved in the 90-odd years since it became a feature of public health studies into infectious diseases. Some countries decided to give herd immunity a chance in the early days of the COVID-19 pandemic, but had to shelve the idea mid-way as more and more infections became apparent, putting the lives of many in danger.

The multiple attempts by different countries towards herd immunity drew criticism from the scientific community, with WHO executive director Mike Ryan saying "we are nowhere close to the levels of immunity required to stop this disease from transmitting". He urged leaders of the world to focus their attention towards suppressing transmission, rather than "live in hope of herd immunity being our salvation".

The case of COVID-19

Herd immunity is typically successful when there is a vaccine available. Numerous vaccine trials are ongoing around the world, with a few candidates having shown promise in their early human trials, which suggests that effective vaccinations would become available by 2021.

COVID-19 has precipitated an unprecedented global crisis. It has even caused many in the scientific community to question their previous methods for countering this contagious disease. What we know so far about the reproductive rate of the virus, or the R0, has also been changing constantly: bringing this rate below one is key to ending this pandemic. An R0 of less than one means that every sick person makes less than one other person ill.

Additional challenges

Many COVID-19 patients present with symptoms that are also common during the flu: fever, cough, shortness of breath. This posed a challenge as the spread of COVID-19 overlapped with the flu season in the northern hemisphere. The rising rates of infection in many cities of the world has overwhelmed the healthcare system of those places, as one found out in places in Italy and Spain, and more recently, in New York, US.

Governments the world over have emphasized the importance of practising social distancing (now called physical distancing) to contain the infection. But in high population density areas and developing countries, maintaining physical distance is not always possible.

Several scientists have dismissed the idea of herd immunity as a feasible method to control the spread of COVID-19. Here are the reasons:

• Herd immunity is usually offered by vaccines.
• Epidemiologists say that if more than half the country's population contracts an infection, they're just sick - it might be fallacious to say that they are in the process of getting immune. That said, some people may, of course, develop an immunity to this strain of the virus.
• According to Gavi, the global vaccines alliance, viruses are constantly mutating. This means that even if a person becomes immune to one strain of the virus, they may fall sick again if they contract another strain of the virus. That said, there is no evidence yet that SARS-CoV-2 - the virus that causes COVID-19 - has mutated to that extent yet. Scientists also say that coronaviruses mutate relatively slowly - which may give us time to develop a vaccine. (Read more: COVID-19 relapse: Can I get this infection twice?)
• According to one estimate, at least 70% of any country's population would have to become infected by COVID-19 (and gain natural immunity to it) to offer herd immunity to the remaining 30%.
  Given that the case fatality rate - or the percentage of people who die after getting this infection - is close to 0.66% (research on this is still evolving), two out of 300 patients would die in the effort to gain this immunity.
  Now, apply that figure to the number of patients in different countries and the mind boggles.
• Those with weak immune systems as a result of underlying illnesses, infant children or older people who cannot be vaccinated may benefit from herd immunity. But a rising number of infected (but otherwise young and healthy) people in a community puts these vulnerable groups at higher risk of contracting the infection as well.

With more than 5.2 million cases of infection and over 84,000 deaths as of 18 September 2020, India's infection ready has been growing rapidly since large-scale restrictions, put in place since 25 March, were slowly lifted and eased since the month of June. The growing cases of infection have also been attributed to increased testing capacity, but epidemiologists and scientists have warned that COVID-19 cases in India will overtake that of the United States soon; India is already the second worst affected country in the world in terms of total infections.

In an interview to a magazine, Dr. T Jacob John, a noted virologist and former head of the Indian Council for Medical Research's (ICMR) virology division suggested the trend of rising cases in India will continue, supporting the projections made by other scientists and researchers studying the Indian scenario. Dr. John also said India would be able to reach herd immunty by early August based on his projections. However, he also pointed out that even if there was a 1% mortality rate in India (it is higher according to Johns Hopkins University data), the death toll in the country could exceed millions due to the sheer population size over time.

Various studies performed around the world have still found a low level of seroprevalence in communities and local populations, which means herd immunity is far from reality anywhere. This has prompted several scientists and experts to say that natural infection just for the sake of achieving herd immunity is unachievable, besides being highly unethical.

Many others have also taken an objection to the term "herd immunity" itself, as it likens human beings to livestock that can be sacrificed for the sake of a larger population. This has prompted many experts to use a more congenial phrase, "population immunity". However, regardless of the political correctness of a term, the fact that herd immunity cannot be achieved without an effective vaccination programme remains the overwhelming reality, because many people would have to lose their lives otherwise.