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Insights into Editorial: Challenges in achieving herd immunity through vaccination

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Context:

On December 1, Health Secretary said that the government has never spoken about vaccinating the entire country against COVID-19.

Adding to that, Director-General of ICMR 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.”

The government’s idea of vaccinating a “critical mass of people” for the purpose of breaking the virus transmission chain is riddled with challenges.

Clarity about Herd Immunity:

  1. Herd immunity (or community immunity) occurs when a high percentage of the community is immune to a disease (through vaccination and/or prior illness), making the spread of this disease from person to person unlikely.
  2. Even individuals not vaccinated (such as new borns and the immunocompromised) are offered some protection because the disease has little opportunity to spread within the community.
  3. Vaccines prevent many dangerous and deadly diseases. In the United States, smallpox and polio have both been stamped out because of vaccination.
  4. However, there are certain groups of people who cannot get vaccinated and are vulnerable to disease: babies, pregnant women, and immunocompromised people, such as those receiving chemotherapy or organ transplants.
  5. For example, the earliest a baby can receive their first pertussis or whooping cough vaccine is at two months, and the earliest a child can receive their first measles vaccine is at one year, making them vulnerable to these diseases.
  6. Herd immunity depends on the contagiousness of the disease. Diseases that spread easily, such as measles, require a higher number of immune individuals in a community to reach herd immunity.
  7. Herd immunity protects the most vulnerable members of our population. If enough people are vaccinated against dangerous diseases, those who are susceptible and cannot get vaccinated are protected because the germ will not be able to “find” those susceptible individuals.

Breaking the transmission chain through vaccination:

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.

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. 

Uneven transmission of virus spreading:

  1. 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.
  2. 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.
  3. 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.
  4. 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.
  5. 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.
  6. It was only in January this year that India achieved 90% coverage of all vaccines to be given in infancy.
  7. 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.

Ethical challenges in identifying the high-priority groups:

  1. 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.
  2. Objective, transparent processes for making priority-setting decisions are extremely important to maintain trust in the vaccination plans.
  3. These should be communicated publicly, including the rationale for the choices, and there should be a mechanism of appeal. Public inputs are crucial.
  4. 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.
  5. Breaking the chain of transmission with partial vaccination of populations is not a concept.
  6. Control of transmission requires either very high levels of coverage or combining vaccination at a reasonable level with well implemented testing and isolation.
  7. The purpose for prioritisation was to protect those at risk of severe disease first and then move to lower risk groups.

Conclusion:

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.

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.

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.

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.