What is herd immunity?

Can the Omicron strain of Corona be defeated?

Is herd immunity a way to combat COVID-19?

What is the latest data on the effectiveness of vaccines against Omicron?

What is herd immunity?

"Herd immunity" - also known as "population immunity" - expresses indirect protection from an infectious disease, and is achieved when a population acquires immunity from that disease, either through a vaccine or previous infection, according to the World Health Organization.

 What is the position of the World Health Organization on “herd immunity” as a means to combat Covid-19?

The WHO says that attempts to achieve "herd immunity" by exposing people to a virus "is scientifically problematic and unethical. Allowing COVID-19 to spread to populations of all ages and health conditions will lead to outbreaks of infection, unnecessary suffering and death."

The World Health Organization supports the achievement of "herd immunity" through vaccination, not by allowing the disease to spread among any population segment, because of the unnecessary injuries and deaths that may result.

Herd immunity to COVID-19 should be achieved by protecting people by vaccination, not by exposing them to the pathogen.

WHO says that to safely achieve herd immunity against COVID-19, a large proportion of the population must be vaccinated to reduce the total number of viruses able to circulate in the population.

One of the goals of pursuing herd immunity is to keep vulnerable groups that cannot be vaccinated (due to certain conditions, such as allergic reactions to a vaccine) safe and protected from disease.

The proportion of those who must be vaccinated to achieve herd immunity varies from one disease to another.

For example, herd immunity to measles requires that 95% of the population be vaccinated, and the remaining 5% is protected because measles outbreaks are interrupted among those vaccinated.

As for polio, 80% coverage is sufficient.

The proportion of the population that must be vaccinated against Covid-19 to achieve herd immunity is still unknown, and it is an important area of ​​research, and the proportion is likely to vary according to society, vaccine, population groups that have priority for vaccination, and other factors.

Why does it now seem that herd immunity due to the emerging corona virus cannot be achieved?

Under this heading, Professor Sheldon H. Jacobson, Professor of Computer Science at the University of Illinois at Urbana-Champaign, in the United States, wrote in Cleveland.com, saying that as cases of Covid-19 virus infection rose during the fall of 2020, discussions were about herd immunity. scattered everywhere.

But despite the availability of effective vaccines, and nearly 50 million confirmed cases reported in the United States, herd immunity is no longer viable for COVID-19, so what happened?

The professor explained that herd immunity is a function of the virus infection.

If the virus is not particularly contagious, the percentage of the protected population that results in herd immunity is lower than if the virus were highly contagious.

The level of infection is recorded in the basic reproduction number, or "R0", pronounced "RNAT" or "RZERO", which indicates the average number of people who transmit the disease to them.

If the basic reproduction number is less than one, the virus will disappear naturally, because fewer people are susceptible to infection.

If it is greater than one, the growth of new infections will rise exponentially.

Herd immunity is a local phenomenon, meaning that a community may have an R-value of less than one.

However, if infected and susceptible people enter the community, the value of RZero will increase, effectively increasing community risk and losing herd immunity.

Several factors have hampered herd immunity over the past year, and are likely to prevent it from occurring anytime soon, namely:


Previous infection does not provide permanent immunity to corona.

Studies indicate that those who have already been infected may have natural immunity for several months, but this protection eventually diminishes, which returns them to the group of those who are exposed to infection, and therefore the idea that the one who was infected and recovered is no longer exposed to Corona, is misplaced.


The delta variant is more contagious than the previous variants, and this has raised the level of those who are immune to achieve herd immunity.

Early estimates put the proportion of those who are immune to herd immunity at 70 percent, but Delta has changed that estimate, pushing the number much higher and beyond the reach of most regions.


Available vaccines provide protection, but they diminish over time, which is why booster doses are now required to maintain immunity.

Moreover, vaccines are more effective in reducing severe disease outcomes, i.e. hospitalization and death, while superinfection - infection of a person who has received the vaccine - continues to occur.

Those with such a superinfection can transmit the virus, keeping the risk of spread high, and preventing the RZero value from staying below one.


New strains create new uncertainties that can alter the calculation of risk for individuals and societies.

The new strain Omicron Much remains to be assessed about its contagiousness and virulence - the degree to which it can cause disease and harm - as well as how well existing vaccines protect against it.


The percentage of those at risk of infection remains very high, with more than 20% of those eligible for vaccination still not being immunized.

Whether they do not have natural immune protection, or choose to remain unimmunized, they contribute to the percentage of the population at risk and at risk, effectively maintaining a high RZ value.

Given the current situation, it is likely that herd immunity will not occur, as the virus tends to become an endemic disease, and this means that effective treatments are critical, so that when people become infected, these products will work to keep them out of hospitals, and protect them from suffering. One of the most harmful effects of the virus is, for some: keeping them alive.

Omicron largely evades immunity from a previous infection or two doses of the vaccine

A new report from Imperial College London reveals that the Omicron strain largely evades immunity built up from previous infections, or two doses of the vaccine.

The report found that the risk of reinfection with an omicron is 5.4 times greater than that of the delta strain, meaning that protection against reinfection from an omicron provided by a previous infection could be up to 19%.

The researchers estimated that the basic reproduction number of the omicron was higher than 3 during the studied period.

The study found no evidence that Omicron is less dangerous than Delta.

This is judged either by the proportion of those who test positive for symptoms, or by the proportion of cases seeking hospital care after infection.

However, hospitalization data is still very limited at this time.

The researchers found a significantly increased risk of symptomatic Omicron compared to delta for those who exceeded the second vaccine dose by two weeks or more, or the booster dose (AstraZeneca and Pfizer) by two weeks or more.

Depending on the estimates used for vaccine efficacy against symptomatic infection from the delta variant, this translates into estimates of vaccine efficacy against symptomatic omicron infection between 0% and 20% after two doses, and between 55% and 80% after a booster dose.

Professor Neil Ferguson, from Imperial College London, said: "This study provides further evidence of the very large extent to which Omicron can evade prior immunity resulting from infection or vaccination. This level of immune evasion means that Omicron poses a major and imminent threat to public health."