“Path to Herd Immunity” to “Path to Normality” is a Mirage

CME INDIA Presentation by Dr. S. K. Gupta, MD (Med), FICP, CFM (France) Clinical Asst. Professor GS Medical College, CCSU, Uttar Pradesh, India. Visiting Consultant, Max Super Specialty Hospital, New Delhi.

“We’re moving away from the idea that we’ll hit the herd-immunity threshold and then the pandemic will go away for good,” – Epidemiologist Lauren Ancel Meyers, executive director of the University of Texas.

Why did Herd Immunity not work for Covid-19?

• More than 10.1 billion doses of vaccine have already been injected word over. Nearly 61.7% world’s population has been vaccinated with at least one shot. And more than 416 crore people have received full vaccination by both doses. But Covid-19 refuses to go from the world.
• The contagion continues to haunt even the countries with more than 90% vaccination coverage like UAE. Why? What happened to Herd Immunity?  Where are we heading to?

Principle of Herd Immunity?

• Herd immunity, or (Community immunity), is said to have kicked in when a large cohort of the population in an area becomes immune to a pathogen, and that level of societal protection is able to contain further spread of disease.
• The concept of Herd Immunity is based on the principle that the microbes use susceptible hosts to travel in society. When with widespread effective vaccination, hardly any people are left in society to carry the infection forward, even the vulnerable remain shielded from the disease, then the Herd Immunity is said to have set in.
•  So, in the herd, while not every single individual is immune, the group as a whole gets an umbrella of protection. The infection rates drop, and the disease wanes off.

When is a community said to have acquired herd immunity?

• When a specific percentage of the population becomes immune to the disease either by vaccination or by infection.
• Depending upon its transmissibility percentage thresh hold required may vary for every pathogen.
• Microbes with higher transmission rates would obviously need a larger percentage of the immune population to be contained. For example, herd immunity against measles requires about 95% of the population to be vaccinated. For polio, the threshold is about 80%.

How do scientists calculate the Herd immunity thresh hold?

• Herd immunity depends upon the reproduction number or R0. The R0 is the average number of people that an infected person can further infect if those people aren’t already immune. The higher the R0, the more people need to be resistant to reach herd immunity.

Why did Herd Immunity experiments fail for Covid -19?

• Because the virus has been constantly evolving to more and more transmissible variants.
• The original Wuhan virus had an R0 of 2.9, requiring about 65% immune population. But soon the Virus changed to the alpha version which had R0 of 4-5 demanding a percentage of 75-80%. Further evolution of virus in Delta Avtar had R0 of 5.1 and in late 2021 came the Omicron with a sky high R0 of 10 -21, requiring immunization of more than 90% of population.

But why is herd immunity not working for countries like India despite more than 95% seropositivity?

• Herd immunity requires permanent robust immunity.  The immune resistance of individuals against Sars CoV-2 in society is going down either by virtue of waning Vaccine Effectiveness (especially mRNA vaccines) or Evolving Virus has been overriding the existing immunity.
• Studies showing that boosters are important in preventing symptomatic infection suggest that vaccine effectivity is going down.

WHO’s vaccination call for vaccination of more than 70% of world population-will it set in Herd immunity?

• WHO set a target for all countries to vaccinate 10% of their populations by the end of September. 56 countries effectively excluded from the global vaccine marketplace were not able to reach this target – and most of them in Africa.
• With vaccine inequality, many countries are at risk of missing the WHO targets of vaccinating 70% of the population of every country by the mid of this year 2022. Though a composite figure of vaccination 70% of world population is likely to be achieved within a month with current rates of vaccination, the figure may remain far from being achieved for certain African nations.

But mass vaccination would still be effective

• Recent observations amply clarify that vaccines protect from severe disease, hospitalization and death. Further mass Vaccination may be able to protect the world from emergence of newer variants.
• So, mass vaccination may not have been a guarantee of immunity from disease but Definitely a surety of safety from death.
• The interplay of Virus and Vaccines may not have been able to achieve herd immunity but has significantly brought down mortality from Covid-19. In fact, Current vaccines have been able to tame the virus to the level of Flu with similar case fatality rates.  

Why we are not able to achieve Herd immunity from Covid -19?

Vaccines produce three types of Immunity

1. Local Mucosal Immunity in form of IgA antibodies which block the entry of the pathogen and are important in curtaining the speed of transmission. We know present Covid-19 vaccines don’t induce local mucosal immunity and hence don’t protect from infection, so the pandemic is likely to continue unless we have effective vaccines that are able to break the chain of transmission even for the future variants. Nasal Vaccines remain the hope of future.

2. Humoral Immunity: IgG antibodies, most commonly talked about immunity, also easy to measure by widely available tests. Certain diseases like Measles and Yellow fever maintain the specific antibody level and hence Herd immunity.  But in Covid-19 the antibody levels drop within 4 to 6 months of vaccination requiring the booster dose so Herd immunity doesn’t set in.

3. Long term Cellular Immunity: Conferred by patrolling T cells that are crucial in long-lasting protection. With are no commercially available tests to measure the same remain most elusive to document in most research papers.  This immunity takes about 7 to 10 days’ time to mount an immune response as the time taken in the production of a fresh crop of antibodies after the antigenic challenge.

The decrease in the number of severe cases and falling mortality rates against Omicron may be attributed to this arm of the immune system. But how long this protection remains effective in case of Sars Cov-2 will be known with time.

As, even this arm of immune response is vulnerable to being rendered ineffective in case the virus evolves to a variant with a very short incubation period, mounting quick complications even before T cells are able to kick in production of antibodies against fresh challenges of virus.

Should people take repeat doses of the Covid-19 vaccine?

• Recommendations are based on evidence, protecting life is most important and repeated doses are not new among vaccines.
• After all, we give two shots against measles during childhood, more than Five doses for DPT and multiple jabs are given against tetanus – now recommended one every 10 years for adults.
• In the same way, some way countries are now giving out a third or even fourth dose of the COVID-19 vaccine in the space of a little over a year after studies showed that vaccine effectiveness was waning.
• With harder to achieve Herd immunity for Covid-19, it is all the more every individual’s responsibility to stay protected and safe by following scientific guidelines.
• With evolving experience and understanding recommendations may change but till then bear the blanket of frequent vaccination.

CME INDIA Learning Points

• Access to vaccines, masks and tests: leading to awkward transition from pandemic to endemic.
• The herd immunity threshold depends: How transmissible is the pathogen & the behavioural and immunological characteristics of the population in which it is spreading (how much they mix and how easily they are infected).
• “As COVID-19 vaccination rates pick up around the world, people have reasonably begun to ask: how much longer will this pandemic last? It’s an issue surrounded with uncertainties. But the once-popular idea that enough people will eventually gain immunity to SARS-CoV-2 to block most transmission — a ‘herd-immunity threshold’ — is starting to look unlikely can slow the virus’s spread. But as these interventions are lifted or varied across populations virus transmission will again accelerate, raising the herd immunity threshold compared to that of a more cautious population. At the same time, social inequities may exacerbate the disproportionate toll Covid has already had on marginalised communities by raising the herd immunity threshold locally.” – Erin Mordecai is an associate professor of biology at Stanford University.

CME INDIA Tail Piece

• “The vaccine will mean that the virus will start to dissipate on its own,” But as new variants arise and immunity from infections potentially wanes, “we may find ourselves months or a year down the road still battling the threat, and having to deal with future surges”- Meyers, Infectious disease expert

• “Herd immunity is only relevant if we have a transmission-blocking vaccine. If we don’t, then the only way to get herd immunity in the population is to give everyone the vaccine,” Shweta Bansal, Mathematical biologist at Georgetown University in Washington DC.

References:

1. Nature 591, 520-522 (2021)doi: https://doi.org/10.1038/d41586-021-00728-2
2. How to Understand “Herd Immunity” in COVID-19 Pandemic. Front. Cell Dev. Biol., 24 September 2020 | https://doi.org/10.3389/fcell.2020.547314
3. Fine, P. E. M., Mulholland, K., Scott, J. A., and Edmunds, W. J. (2018). “Community protection,” in Vaccines, 7th Edn, eds S. A. Plotkin, W. A. Orenstein, P. A. Offit, and K. M. Edwards (Amsterdam: Elsevier), 1512–1531.
4. Haralambieva, I. H., Kennedy, R. B. I, Ovsyannikova, G., Schaid, D. J., and Poland, G. A. (2019). Current perspectives in assessing humoral immunity after measles vaccination. Expert Rev. vaccines 18, 75–87. doi: 10.1080/14760584.2019.1559063

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