Challenges in achieving herd immunity through vaccination

Breaking the chain of transmission with a partial vaccination of populations may not be possible

Updated - December 06, 2020 11:16 am IST

Published - December 05, 2020 07:38 pm IST

Promoting equity:  For a disease where everyone is equally affected, immunisation should be available to all when vaccines are ready, says Dr. Gagandeep Kang.

Promoting equity: For a disease where everyone is equally affected, immunisation should be available to all when vaccines are ready, says Dr. Gagandeep Kang.

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On December 1, Health Secretary Rajesh Bhushan said that the government has never spoken about vaccinating the entire country against COVID-19. Adding to that, Director-General of ICMR Dr. Balram Bhargava said: “If we’re able to vaccinate a critical mass of people and break virus transmission, then we may not have to vaccinate the entire population.”

But the government’s idea of vaccinating a “critical mass of people” for the purpose of breaking the virus transmission chain is riddled with challenges. Unlike the mRNA vaccines from Pfizer and Moderna, the interim analysis of Phase-3 trial of the Oxford vaccine has shown promise to prevent virus transmission. But the prime objective of the Phase-3 trials was to test the ability to prevent severe disease. It is only when the final analysis of Phase-3 data of the Oxford vaccine and other vaccines in development become available can the government be certain of breaking the transmission chain through vaccination.

On the issue of the government shying away from universal vaccination, Dr. Anant Bhan, a researcher in global health and bioethics, says in an email: “We need a clear enunciation of the plan. If it is to vaccinate a subset of the population, we need to see a scientific rationale for that choice, and how the decision to select the subset was made.”

“For a disease where everyone is equally affected, immunisation should be available to all when vaccines are ready. It is important to remember that vaccines are a tool to promote health equity,” Dr. Gagandeep Kang, Professor of Microbiology at CMC Vellore says in an email. “If the goal is to achieve herd immunity, we will need about 70% of the population to be covered. Unlike other infections where groups which have high risk of transmission can be identified, the task is complicated for SARS-CoV-2.”

Uneven transmission

For instance, the levels of immunisation needed for herd immunity are determined by how the virus spreads in the population, and makes the assumption that spread is homogenous. But SARS-CoV-2 virus spread exhibits a high level of uneven transmission. This is the reason why there have been a number of super-spreading events where some infected individuals spread the virus to very a large number of people while most infected individuals transmit the virus only to a few or none.

While targeted vaccination of sex workers and injection drug users, where more HIV cases are seen, can help prevent the virus from spreading to the general population, such concentration of cases is not seen in the case of coronavirus. “Unlike HIV, for SARS-CoV-2 we do not have any such groups. Even healthcare workers with PPE now have low levels of transmission so much so that many countries are thinking about not prioritising them,” says Dr. Kang.

Considering that two doses of the vaccine are needed for full protection and increased vaccine hesitancy particularly as the vaccine development and testing are seen to be rushed, achieving herd immunity of 70% to break the chain would be challenging. According to Dr. Kang, it was only in January this year that India achieved 90% coverage of all vaccines to be given in infancy. If there is a drop in vaccine coverage in children beyond their first year of life in the immunisation programme, it becomes particularly difficult in the case of SARS-CoV-2.

Aim high

“With COVID-19 vaccines, we will also need to reach age groups which are not currently targeted as part of the large-scale immunisation efforts. This will be a design, logistics and implementation challenge. All of these aspects are important to consider while considering the country-wide COVID-19 immunisation plans,” says Dr. Bhan. Dr. Kang adds: “So if anyone were to ask me, I would say aim high from the start, even if coverage is in phases.”

Clinical trials test the efficacy of the vaccine, while the actual effectiveness of the vaccine will be known only when a large number of people are vaccinated post-licensure. Also, the duration of protection is not known and hence how frequently the vaccine has to be administered remains unknown. It is critically important to understand these to make sure that no resources are diverted from existing immunisation programmes that need to continue, she says.

Ethical challenges

Considering that the government has already listed out the high-priority groups that will receive the vaccine, the issue of choosing other sections of the population that needs to be vaccinated to achieve herd immunity will be ethically challenging. “Objective, transparent processes for making priority-setting decisions are extremely important to maintain trust in the vaccination plans. These should be communicated publicly, including the rationale for the choices, and there should be a mechanism of appeal. Public inputs are crucial,” says Dr. Bhan.

Incidentally, the intent behind identifying the high-priority groups to receive the vaccine first was to safeguard them from severe disease and not to break the virus transmission chain. “Breaking the chain of transmission with partial vaccination of populations is not a concept I understand. Control of transmission requires either very high levels of coverage or combining vaccination at a reasonable level with well implemented testing and isolation,” comments Dr. Kang. “I thought the purpose for prioritisation was to protect those at risk of severe disease first and then move to lower risk groups.”

Updating the evidence

Another contentious area is the question of vaccinating those who have already been infected. “The immunity offered by the natural infection is probably long term. Till date, we are not aware of the added benefits or risks of vaccinating the people who are already infected. It is time we start doing some quick trials to update the evidence based on the contextual needs,” says Dr. Giridhara Babu, Epidemiologist at Bengaluru’s Public Health Foundation of India.

Citing the Oxford vaccine trial results that showed 90% efficacy in those who received half dose-full dose regimen, Dr. Babu feels those who have had mild or no symptoms either due to low viral inoculation dose or with better immune response, or both might respond better with vaccination.

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