New Delhi: The central government’s announcement that vaccinating a “critical mass” of the population against Covid-19 is among the options on the table has left experts wondering about the feasibility of any such initiative.
Vaccinating a “critical mass” would mean giving the Covid vaccine to enough people that the chain of transmission is broken — or effecting “herd immunity” in the population.
However, experts doubt that is possible, at least for now, since the current lot of promising Covid vaccines aren’t the ‘sterilising’ type. This means that they won’t stop the novel coronavirus from entering the cells of a vaccinated individual — all they promise to do is prevent the infection from taking on a severe form.
These vaccines, they say, would at best shrink a Covid-infected person’s window of infectivity — the amount of time they serve as a carrier of the virus — rather than closing it altogether. Breaking the chain of transmission becomes difficult in such a situation, experts say.
The central government announced its “critical mass” plans at a Union Health Ministry briefing this week where it took a U-turn on PM Narendra Modi’s promise that “everyone will be vaccinated”.
In an interview to The Economic Times in October, PM Narendra Modi had said, “First and foremost, I would like to assure the nation that, as and when a vaccine becomes available, everyone will be vaccinated. None will be left behind. Of course, initially we may focus on protecting the most vulnerable and the frontline workers. A National Expert Group on Vaccine Administration for Covid-19 Vaccine has been constituted to chart the way forward.”
However, at the briefing, Union Health Secretary Rajesh Bhushan was emphatic that the government had never made a Covid-vaccine-for-all commitment.
“The government never spoke about vaccinating the entire country. It is very important to get the factual information on these things,” he said.
Speaking after him, Union Secretary for Health Research and Indian Council of Medical Research (ICMR) Director General Dr Balram Bhargava said, “We may or may not need to vaccinate all. If we can reach a critical mass before doing so, then it is not required. That is why measures like masks etc will need to continue.”
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What is a ‘critical mass’ for Covid?
‘Critical mass’ for Covid is not known for certain and is currently a matter of conjecture, say experts. Decisions based on this figure, they add, will have to be driven by the past and the future trajectory of the pandemic.
In an article published in The Lancet in September, researchers from the University of Hong Kong wrote that “perhaps about 25-50 per cent of the population would have to be immune to the virus to achieve suppression of community transmission”.
Looking at the performance of various vaccine candidates, including in preclinical studies, they said: “These observations suggest that we cannot assume Covid-19 vaccines, even if shown to be effective in reducing severity of disease, will reduce virus transmission to a comparable degree. The notion that Covid-19-vaccine-induced population immunity will allow a return to pre-Covid-19 ‘normalcy’ might be based on illusory assumptions.”
Dr Anurag Agarwal, director of the Delhi-based Institute of Genomics and Integrative Biology, which is overseen by the Union Ministry of Science & Technology, is more inclined to suggest 50 per cent as the mark for herd immunity, but admits it is difficult to say so for certain.
“The ‘critical mass’ is an inexact number as of now but, looking at cities like Delhi and Mumbai, it is likely to be in the range of 50 per cent or so — one confirmation will be if Delhi, for example, does not have another spike in the coming days,” he said.
The August sero survey in Delhi showed that about 30 per cent of the population has been exposed to the novel coronavirus. After the third wave — which has been bigger than the past two waves — it is safe to assume that the figure could have reached 50 per cent, Agarwal added.
“Risk levels of various population groups will also have to be taken into account. The scientific way of doing vaccinations could be to test people for antibodies before giving them the shot but that may not be feasible. Although if the availability is too small, it may still be an option,” he added.
While the central government has identified priority groups like healthcare workers, elderly and comorbid people for vaccination, children may not feature anywhere in the list as most vaccines have only been tested on adults. This means around 40 per cent of the population is straightaway not likely to get the vaccine.
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Is herd immunity an option now?
Two experiments conducted early on in the pandemic have queered the pitch for the ‘herd immunity’ approach.
The first was in the UK, when Prime Minister Boris Johnson’s plan to let the virus run through the population was halted by immediate backlash.
The second was in Sweden, which bucked the lockdown approach to adopt what is now known as Covid-appropriate behaviour in the quest for herd immunity. It seemed to be successful initially, but was eventually overwhelmed by infections and was making headlines for its handling of the pandemic.
Anders Tegnell, Sweden’s state epidemiologist, now says it’s still not clear to what extent transmission rates are reduced when more people have been exposed to the virus.
However, with the UK rolling out a vaccine for public use by next week, and the Russians looking at a January launch for their candidate, the herd immunity conversation may get a fresh lease of life.
The World Health Organization (WHO) describes herd immunity or ‘population immunity’ as a “concept used for vaccination, in which a population can be protected from a certain virus if a threshold of vaccination is reached”.
Herd immunity, it says, is “achieved by protecting people from a virus, not by exposing them to it”.
“Vaccines train our immune systems to create proteins that fight disease, known as ‘antibodies’, just as would happen when we are exposed to a disease but — crucially — vaccines work without making us sick. Vaccinated people are protected from getting the disease in question and passing it on, breaking any chains of transmission,” it states.
However, the current crop of Covid vaccines that have been approved or are at an advanced stage of trial do not aim to give foolproof protection against the disease as, for example, is done by some influenza vaccines that are given nasally and do not allow the pathogen to enter the body at all.
“For the initial infection to be blocked, we need inhaled mucosal vaccines which produce secretory IgA antibodies. Those vaccines are still under early evaluation. The injectable vaccines may limit the period of infectivity by preventing a severe infection but not fully guarantee stoppage of transmission even for a limited period from a vaccinated person who acquires the virus,” said Dr K. Srinath Reddy, president of the Public Health Foundation of India (PHFI) and member of the International Steering Committee of the WHO Solidarity Trial, which is aimed at finding an effective treatment for Covid-19.
“For that reason, it is difficult to predict what percentage of people need to be immunised to be assured of breaking the chain of transmission. If the main goal is to prevent severe illness and death, as it must be, all vulnerable persons in the population must be prioritised for immunisation,” he added.
“Disrupting the chain of transmission through limited immunisation coverage is a possible but uncertain outcome.”
The systemic vaccines on the frontlines, including the two mRNA vaccines by Pfizer and Moderna (the former has now been approved in the UK), are positioned to reduce the severity of the Covid-19 infection, not the risk of acquiring infection per se, he said.
“That is the clinical outcome which they are reporting to the regulators from the trials. It is still possible that they will not be able to prevent the initial infection of the respiratory tract or the transmission of the virus to others from that person,” he said.
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