BENGALURU: Herd immunity. It’s a term that’s now in news again as the country began its nationwide rollout of Covid-19 vaccine on Saturday. The goal of vaccinating the entire population, according to the Union Health Ministry, would be to help states reach a level of herd immunity.
What that means is that a majority of the population is immune to an infectious disease, slowing down the rate at which it spreads. But the launch of the vaccination campaign also has reignited a debate on whether herd immunity is possible with only the help of vaccines. No, said Dr Vishal Rao, Head and Neck Oncologist, who was a pioneer in Karnataka to get permission to perform plasma therapy trials on Covid patients.
According to recent studies, considering that R0 (read as R Naught) of the virus is 3, about 67 to 75 per cent of our population should be immune to the virus. This means that four out of five people would not get sick if they encountered someone who has been infected, possibly unable to spread the disease any further, he explained. Dr Rao argued, “There’s a problem with this perfect image of herd immunity.
It is not as simple as it seems. The studies have shown that there is no exact number of R0 and it can vary depending on the population and the virus.” According to him, the R0 for SARS-CoV-2 is especially uncertain. In some regions, it is as low as 2 but in some as high as 6, he explained. Another senior doctor from a renowned government hospital in Bengaluru, on condition of anonymity, said, “Herd immunity is achievable with a vaccine with 70 per cent efficacy against transmission, but would require high coverage in the 18-35 year group in particular, which appears to be a great contributor to the transmission.
But then in India, we are looking at older and vulnerable population for containing the spread. Also, the efficacy studies of the two vaccines are not known yet.” Dr Vishal Rao has posed some questions to vaccine manufacturers and the central government on the efficacy of the vaccine itself, which according to the World Health Organisation is an important factor to attain herd immunity. He said, “The existing data from the vaccine manufacturers only provides evidence of protection against a progressive Covid disease, but not against person-to-person transmission.”
Dr Soumya Swaminathan, Chief Scientist at WHO, has made it clear, “Even as vaccines start protecting the most vulnerable, we’re not going to achieve any levels of population immunity or herd immunity in 2021. Even if it happens in a couple of pockets, in a few countries, it’s not going to protect people across the world.” Experts claimed that even if Karnataka reaches a 70 per cent overall vaccination rate, it can still leave large pockets of the population prone to outbreaks.
“This is the scenario that public health leaders have been warning about for years with diseases like measles. Different communities within the state will also have different herd immunity thresholds. That is because R0 for the virus will be different depending on how populations interact. Dense communities where people live closer together, giving the virus a better chance of spreading, will have higher herd immunity thresholds. Less dense communities will have lower figures,” explained a senior doctor from Mangaluru.
A study confirming this was published in 2019 in the journal “Emerging Infectious Diseases”. It said, “Because R0 is a function of the effective contact rate, the value of R0 is a function of human social behaviour and organisation, as well as the innate biological characteristics of particular pathogens.”
Some experts recently published articles stating that there is “scientific evidence that natural immunity attained after recovery from a Covid infection is effective and long-lasting and hence it may be dangerous to vaccinate all.”
In his recent report, Jay Bhattacharya, a professor of Medicine at Stanford University, explained, “The immune system responds to infection by various mechanisms, including the production of specific antibodies, T-cells, and B-cells to protect nearly every recovered Covid patient from reinfection. After almost a year of pandemic, globally, only 31 cases and two deaths have been identified as reinfections, out of the 90 million Covid cases and likely hundreds of millions of infections worldwide.”
But many senior doctors and researchers like Ananth Bhan, a bioethic researcher, argued against this theory of herd immunity only through natural infection. He said, “We don’t know enough about the immunity in a natural infection. It has been shown that the antibody in a Covid-infected patient has been short and a patient is susceptible to infection again.” It is definitely not worth taking the risk with this in a country where there are so many senior citizens and those with comorbidities, the experts argue.
“We are still learning about this virus and also the immunopathology of vaccine as one of the responses. There is no one size fits all. Covid precautions like wearing masks, social distancing, washing of hands are important. Though the number of infections is falling, we cannot test the antibody levels of each person who has been vaccinated. It is better to use vaccination as a prime strategy to prevent infection,” Dr Bhan said.
Dr Manjunath, Director of Jayadeva Institute of Cardiology and member of Covid Advisory Committee who has also treated many Covid patients, said, “We have seen that immunoglobin G (IgG) levels of those who have recovered from Covid have been very low. Vaccinating even those will definitely boost further immunity. Though we know that quite a considerable percentage of the population in the state would have already been infected, it is advisable to take the vaccine.”
Senior doctors said that the concept of herd immunity has proved wrong not only in countries like Sweden but also in cities like Mumbai, Delhi and now in Kerala. Dr Shashikiran Umakanth, Professor and HoD, Nodal Officer for Covid, Dr TMA Pai Hospital, Udupi said, “Though we wish that we have natural immunity, we cannot take the disease for granted. Also, the vaccination drive is not yet powered to achieve herd immunity. We have to be clear that this drive is to give protection to the vulnerable population.”
Some epidemiologists who have analysed the health ministry’s goal of giving shots 60-80 per cent of the population, which translates to about 80 to 100 crore people over the next few months, said, “This means the government machinery will have to be mobilised to carry out over 25 lakh vaccinations per day, as opposed to three lakh-odd shots being given during Phase 1.”
A senior virologist from Dakshina Kannada said, “The government may be able to achieve herd immunity by August when the country will have 22 per cent of the population vaccinated. But to achieve herd immunity or reach closer to that target, we will still need to cover over 50 crore more people. Post-August 15, if we continue at the same pace, it will take us eight months to cover 90 crore people. So if we double the pace of the vaccination in mid-April 2022, we can reach the target by the end of that year.”
But Dr Sudarshan Ballal, Chairman, Manipal Hospitals, said, “Considering that 50 per cent of them would have already been infected and if we are vaccinating even another 30 per cent of the population that is vulnerable, I feel we would have achieved the herd immunity target.” The debate on natural immunity versus herd immunity through vaccines has kicked up a storm in social media circuits.
Senior doctors, however, said that the trajectory of the disease is in people’s hands and we should all contribute to stop the pandemic. “Getting vaccinated doesn’t mean that we don’t wear masks, follow social distancing and maintain hand hygiene. Covid is not over yet. A vast majority of people are still vulnerable. We have to protect them,” said Medical Education and Health Minister Dr K Sudhakar.
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