But what is herd immunity and how can it be achieved?

Herd immunity can help reduce the risk of sudden spikes in cases, even if the disease cannot be completely eliminated, said Dr Kirti Sabnis, infectious disease specialist, Fortis Hospital Kalyan and Mulund. “It helps to protect against sudden spikes in disease cases, particularly in patients with low immunity due to factors such as age, cancer, HIV, or if patients are undergoing chemotherapy. However, it takes time to develop herd immunity,” Dr Sabnis told indianexpress.com.

Experts suggest that herd immunity can be achieved in two ways. One is naturally, when everyone tends to develop antibodies against the disease. That is when most of the population gets infected or sick by the virus and thus acquires natural immunity by producing antibodies against that pathogen. Another way is through mass vaccination of people — this provides artificial immunity, explained Dr Ravi Shekhar Jha, director and head, pulmonology, Fortis Hospitals, Faridabad.

“This is important in breaking the chain of any disease, which stops growing when a person is immune. However, it appears that with Covid, it is difficult to achieve herd immunity since the virus is highly mutagenic and keeps on mutating to more or less virulent forms. We have seen that many mutated variants had immune escape potential, which means the existing antibodies could not recognise it,” Dr Jha told indianexpress.com.

World Health Organization (WHO), on its website, states that it is also looking into whether the strength and length of immune response depend on the type of infection a person has: without symptoms (‘asymptomatic’), mild or severe. Even people without symptoms seem to develop an immune response. However, WHO supports herd immunity through vaccination and not by allowing disease to spread through any segment of the population, as this would result in “unnecessary cases and deaths”.

Herd immunity is usually achieved for viruses that do not mutate and are stable. (Source: Express Photo by Amit Chakravarthy)

To safely achieve herd immunity against Covid-19, a substantial proportion of a population would need to be vaccinated, lowering the overall amount of virus able to spread in the whole population, it states.

Working toward herd immunity also ensures keeping vulnerable populations safe and disease-free who are unable to receive vaccinations (for example, due to medical issues like vaccine allergies), added Dr Arunesh Kumar, HOD department of pulmonology and respiratory medicine, Paras Hospitals, Gurugram.

Agreed Dr Jha and said, “Complete vaccination, including booster dose is the key. We may have to keep on repeating booster dosing if there is any fresh spike of cases in India.”

Currently, China is facing challenges with herd immunity due to prolonged lockdowns and questions about the efficacy of vaccines, which have both contributed to its decrease, Dr Sabnis said. “It is important to note that herd immunity can also decline if there are significant new mutations that allow for the transmission of the virus,” Dr Sabnis added.

Attributing less testing and delayed winter as the major reasons for no fresh spike in cases in India, Dr Jha said, “Fortunately, India has not seen much rise in cases, majorly because of not testing but also evident by the fact that we are not seeing complicated pneumonia cases in this weather. However, this year, winter is also delayed and we need to keep our fingers crosses till January 2023 end.”

As opposed to “last December’s 5.3 per cent confirmed cases we saw only 0.1 per cent cases related to Covid [this time],” said Dr Gowri Kulkarni, head of medical operations, MediBuddy. “This reduction could be attributed to decrease in virulence of the current covid strain, increased vaccination, decrease in testing and development of herd immunity,” Dr Kulkarni said.

According to Dr Kumar, it is unknown what percentage of the populace needs to receive the Covid-19 vaccine (and booster) before herd immunity can start to develop. “This is a crucial area for research, and it will probably differ depending on the community, the vaccine, the populations that are given the highest priority for immunisation, and other elements,” said Dr Kumar.

Dr Kulkarni added that “though the numbers show a decline in India, with the surge of covid cases world wide especially in China, Korea, Japan and USA, we need to be alert”. “The latest directive from the ministry of health and family welfare requesting the states to send positive cases for genomic sequencing is a step towards this,” Dr Kulkarni said.

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