Herd Immunity for COVID-19—Hope and Caution

From the first reports of a cluster of pneumonia cases in Wuhan, China in December 2019, the coronavirus pandemic has led to over 102 million reported cases and over 2 million deaths globally since the start of the pandemic. The pandemic has caused the largest global economic contraction since the Great Depression began in 1929.

It is estimated that by the end of 2021, about 150 million more people could be pushed into extreme poverty. In addition to its immediate effects, income losses, disruptions in health services, and school closures are likely to generate long-term negative impacts resulting in loss of human capital. 

That’s why the World Bank Group will build on its initial COVID-19 response with $12 billion to help developing countries purchase vaccines, set up vaccination systems, and move quickly to vaccinate their people.

Hopes for any economic recovery are pinned on the successful and quick roll out of COVID-19 vaccines. In what can be described as nothing less than a true triumph of science, vaccines have moved from concept to testing and roll out in record time. Yet, the recently released excerpt from the Independent Panel for Pandemic Preparedness and Response highlighted a series of failures which led to lost opportunities to apply basic public health measures and help reduce the effects of the pandemic.

Governments all over the world are eagerly hoping on the protection offered by vaccines to return to the normalcy of the pre-COVID times. There are several reasons why this hope should go hand in hand with a healthy dose of caution.

Immunization coverage required to achieve herd immunity   

As countries roll out vaccination, one of the obvious target governments are aspiring to reach is a threshold of immunity in the population to confer “herd immunity”.  Herd immunity occurs when a large portion of a community (the herd) becomes immune to a disease, making its spread from person to person unlikely. As a result, the whole community becomes protected — not just those who are immune.

Researchers have attempted to estimate the “Ro” or the basic reproductive number which is the number of secondary cases generated by one infected individual in an otherwise fully susceptible, well mixed population, and the “Rt” or effective reproductive number which is the number of secondary cases arising from one infected individual calculated using real-life data from testing and disease surveillance. Using the value of Rt, we can then calculate the “Pcrit” or the minimum (‘critical’) level of population immunity to halt the spread of infection in that population.  

An early estimate of the rate of spread (Rt) of the first variant of SARS-CoV-2, during the early stages of the epidemic was between 2.5 and 3. The corresponding protective immunity level (Pcrit) for this value of Rt is 67%. Assuming a vaccine efficacy of 90%, we need to vaccinate approximately 74% of the population to stop the virus from advancing further.

What else needs to be done to reach and sustain herd immunity?

The trillions of dollars lost by the global economy, as a result of the pandemic, has helped governments realize that not investing in vaccines is no longer an option. The COVAX facility, the global mechanism for pooled procurement of COVID-19 vaccines, and financing from the World Bank and other partners have ensured the availability of resources to support the equitable distribution of vaccines.

While some countries are reportedly facing vaccine shortages, it is hoped that manufacturers will be able to successfully enhance production capacity over the coming weeks and months.  However, even if the funding is assured for countries to purchase substantial quantities of vaccines, reaching and sustaining herd immunity is rife with challenges and requires consideration of certain practical issues. 

• Achieving above 70% coverage would not be easy. If we consider that the currently approved vaccines do not target children below the age of 16 or 18, (which is already above 20% of the population in most country contexts), countries would need to immunize all those remaining with very high levels of coverage.

• Not knowing how long the immunity conferred by vaccination will last is a definite concern. This makes it very difficult to calculate the period or timeline for achieving herd immunity. Vaccine supply pipelines and country implementation capacity would necessitate immunization of the population in phases.  Those receiving the dose in the first phase may no longer contribute to the pool of protection if their immunity has waned by the time the last phase of immunization has been completed. Hence, like Flu shots, countries may have to consider repeating doses after a certain period. 

• The emergence of new variants will also complicate the situation further. Even if approved vaccines remain effective against new variants, higher rates of infection would mean higher Rt values and hence higher coverages to be achieved for herd immunity. 

Human behavior needs to be factored in

Of course, the big worry is that human behavior will also influence the force of transmission. While COVID-19 vaccines have shown to clearly prevent infections, researchers need more time to figure out whether they prevent transmission by conferring what is called as “sterilizing immunity”.

In other words, a vaccinated person might still be able to spread the virus, even if they don’t feel sick. Hence, until we are sure of this, we will need to continue relying on the use of masks and social distancing to limit the spread of infection. Yet, vaccines offer the only hope today for restoring normalcy. Assuming the availability of vaccines, countries need to move full steam ahead to vaccinate as many people as they can, as quickly as they can.

Aiming for herd immunity is feasible but should be approached with caution and a realistic timeframe based on scientific evidence. It will also require a close observation of how vaccination is influencing the infection rates in countries.

Thinking long-term

Countries need to urgently consider complementing their short-term vaccination plans, aimed at protecting the health system and at-risk groups. This should include putting in place a more long-term approach for achieving and sustaining high levels of population immunity involving a mix of interventions such as vaccination (and repeat vaccinations if needed), use of masks and social distancing, effective testing and tracing; along with building health system resilience.  

The understanding of what and how much time it will take to achieve and sustain herd immunity will have important policy implications for countries. Prioritization of vulnerable groups, who have often been last to receive public health interventions, would be all the more crucial. If these groups miss out on the opportunity for immunization, they would face a higher risk of infection for a longer duration until herd immunity is achieved.

Meanwhile, the World Bank has been supporting countries with unprecedented social protection measures to respond to the Covid-19 pandemic. These will have to be extended for longer periods or else the poorest will inevitably fall between the cracks.

As societies hope to rebound, combining realistic expectations of achieving herd immunity, based on evidence, with the risks of transmission would help policy makers make informed choices of the strategic sectors and services of the economy to prioritize for reopening in the interim and which to defer for later.