Explained: What is Covid herd immunity and is it possible to achieve it?

New Delhi, Apr 19: Covid-19 cases in India have spiked in the last few days, clearly indicating that the virus has not gone away. With cases in neighbouring China witnessing an upward trend, the fears of the fourth wave have returned.

Experts say that vaccination is the only long-term solution to the Covid-19 crisis to obtain herd immunity. So, what is herd immunity? Is it possible to achieve herd immunity for SARS-CoV-2?

Herd immunity, also known as 'population immunity', is the indirect protection from an infectious disease that happens when a population is immune either through vaccination or immunity developed through previous infection.

The disease gradually disappears from a population once herd immunity is achieved and may result in eradication or permanent reduction of infections to zero if achieved worldwide.

How herd immunity is achieved?

The World Health Organization (WHO) supports achieving 'herd immunity' through vaccination, not by allowing a disease to spread through any segment of the population, as this would result in unnecessary cases and deaths.

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 on the pathogen, breaking any chains of transmission. Visit our webpage on COVID-19 and vaccines for more detail.

To safely achieve herd immunity against COVID-19, a substantial proportion of a population would need to be vaccinated, lowering the overall amount of virus able to spread in the whole population. One of the aims with working towards herd immunity is to keep vulnerable groups who cannot get vaccinated (e.g. due to health conditions like allergic reactions to the vaccine) safe and protected from the disease.

The percentage of people who need to be immune in order to achieve herd immunity varies with each disease. For example, herd immunity against measles requires about 95% of a population to be vaccinated. The remaining 5% will be protected by the fact that measles will not spread among those who are vaccinated. For polio, the threshold is about 80%. The proportion of the population that must be vaccinated against COVID-19 to begin inducing herd immunity is not known. This is an important area of research and will likely vary according to the community, the vaccine, the populations prioritized for vaccination, and other factors. Achieving herd immunity with safe and effective vaccines makes diseases rarer and saves lives.

Can Exposing to Covid Help Achieve Herd Immunity?

Attempts to reach herd immunity through exposing people to a virus are scientifically problematic and unethical. Letting COVID-19 spread through populations, of any age or health status will lead to unnecessary infections, suffering and death. The vast majority of people in most countries remain susceptible to this virus. Seroprevalence surveys suggest that in most countries, less than 10% of the population has been infected with COVID-19.

What Percentage of People Should get Vaccinated to Achieve Herd Immunity?
As per the experts, herd immunity would require around 80-90% of the population to have COVID-19 immunity, either through prior infection or vaccination.

"It's still unclear exactly how many people will need to be vaccinated in order to achieve herd immunity to COVID-19, but experts estimate that it will take at least 70% of the population - with some estimates ranging as high as 90%," says Dr. Ashley Drews, medical director of infection prevention and control at Houston Methodist.

Is it possible to achieve herd immunity?

Experts say that achieving herd immunity in the case of Covid-19 is unlikely to happen as vaccination might be effective only against a particular variant. If a person is infected with Delta variant there is a possibility of getting infected with the Omicron variant, says Thomas Russo, M.D., professor and chief of infectious disease at the University at Buffalo in New York.

The virus can still live in animals and spread to people. "Even if we're able to manage infections in humans-which we won't really be able to-20-plus animals can support replication of the virus. That's another huge barrier," he said.

For a virus as contagious as the delta variant, 98% of the population would need to be vaccinated if the vaccines we have could prevent 85% of transmission of the virus, some experts claim. Whether we achieve herd immunity or not, getting vaccination is the solution we have at this stage.