- Experts say that the point where we achieve herd immunity varies from disease to disease.
- The more contagious a virus is, the greater the proportion of the population that needs to be immune to the disease to stop its spread.
- Tested and approved COVID-19 vaccines are the most effective and safest way to get to herd immunity.
‘Herd immunity’, also called population immunity, is indirect protection from an infectious disease that happens when a population is immune, either through vaccination or from previous infection, according to the World Health Organization (WHO).
Experts say that the point where we achieve herd immunity varies from disease to disease, but the more contagious a disease is, the greater the proportion of the population that needs to be immune to the disease to stop its spread.
Why natural infection may not lead to herd immunity
Tested and approved COVID-19 vaccines are the most effective and safest way to get to herd immunity, according to experts.
Dr. Nikhil Bhayani, an infectious diseases physician advisor at Texas Health Resources, told Healthline that technically herd immunity can be reached if enough people develop COVID-19 and then have antibodies to the virus. However, that type of immunity is unpredictable compared to vaccines.
“The caveat is… for how long is the natural antibody good to prevent re-infection?” he said. “Also, more than 70 percent of the population would need to be infected to achieve herd immunity, which can lead to serious complications including death.”
But Bhayani warned that COVID-19 has severe consequences that make relying on natural immunity unacceptable compared to vaccines that have been tested and continue to be studied.
Vaccination stops disease spread
The WHO recommends varying degrees of vaccine uptake to achieve herd immunity depending on the disease.
“If you are coughing and sneezing, and the droplets reach someone who is susceptible, then the virus will keep spreading,” Ellen Foxman, MD, PhD, Yale Medicine pathologist, and expert in respiratory viruses, said in a statement. “But if the virus reaches someone who has immunity, it is like hitting a wall. The virus can’t go any further.”
The more infectious a disease, the more people need to be vaccinated.
For example, to keep measles from spreading experts advise about 95 percent of the population be vaccinated.
For polio, that threshold is reached at about 80 percent of the population.
But the SARS-CoV-2 virus has mutated into more infectious versions, so experts are not sure exactly how many people will need to be vaccinated to reach herd immunity globally.
“Regardless of what infection we’re talking about, the number of people who need to be immunized, or infected, to get to herd immunity depends on how contagious the condition is,” said Michael Grosso, MD, chief medical officer and chair of pediatrics at Northwell Health’s Huntington Hospital in Long Island, New York. “The more contagious, the more people need to be immune.”
When might the U.S. reach herd immunity through vaccinations?
In the United States, about 53 percent of people are fully vaccinated against the disease. That is far lower than the estimated 70 to 90 percent vaccination rate we’re estimated to need to potentially hit herd immunity.
At the current pace of vaccination, the United States could hit 70 percent vaccination by November 10 and 95 percent by April 10, according to a model by the New York Times. Although the 95 percent could only happen after children under 12 are authorized to get the vaccine.
Experts have been worried that vaccine hesitancy could leave the U.S. unable to reach herd immunity. It may also be why the Biden administration issued new rules that would require private companies with over 100 employees to ensure their employees are either vaccinated or undergoing weekly testing.
“An estimated 30 percent of the country is hesitant to receive a vaccine according to recent polling data,” said Hannah Newman, MPH, CIC, director of epidemiology at Lenox Hill Hospital in New York. “Though it’s theoretically possible to improve this number, it will be challenging to reach the percentages needed in order to attain herd immunity, at least in the immediate future.”
How pandemic could end without herd immunity
Newman said some experts have ceased using herd immunity as an end goal, and rather have shifted to the mindset that COVID-19 will persist as an inevitable but manageable threat.
“If so, we will still expect to see pockets of infection, but in smaller quantities,” she said.
Newman explained that even if it’s unlikely the virus can be completely eliminated, increasing vaccinations as much as possible will assure that we are doing all we can to keep infections mild and to keep hospitalizations and deaths at a minimum.
“It will also allow for the safe easing of restrictions and return to more “normal” pre-pandemic activities,” she said.
Some countries without any vaccine access
Grosso expressed sadness and frustration that while vaccine hesitancy is a U.S. problem, there are countries where people want the vaccine, but can’t access it.
“It is incredibly sad and frustrating that internationally the opposite is true — too few people immunized because there is too little vaccine,” he said.
Grosso warned that the more susceptible people there are, the greater the likelihood that disease transmission will continue.
Herd immunity can help prevent new dangerous variants
Bhayani cautioned that new variants mean the bar to reach herd immunity could get even higher, and it’s critical that more people get vaccinated.
“If everyone does their part in getting the vaccine, and boosters once available, we can strive to reach herd immunity,” said Bhayani.
Grosso emphasized that this is the only way out of the pandemic, and the longer it takes to reach herd immunity through vaccination, the greater risk of dangerous variants emerging.
“The longer this takes, the likelier it is that new variants will continue to emerge, some of which may be more transmissible or more resistant to the current vaccines or both,” he warned. “If we can’t get more people immunized, it is possible that this vicious cycle will continue for quite a while.”
The bottom line
Herd immunity is reached when there are enough people immune to a disease that it can’t easily travel from person to person.
Experts say that vaccination is the way to accomplish this for COVID-19 with as little loss of life as possible, and that a vaccination rate above 70 percent will be needed to end the pandemic. The delta variant is more infectious and has led to a major surge of COVID-19 in the U.S. potentially complicating our ability to reach herd immunity.
They also say that vaccine hesitancy is slowing down progress toward herd immunity and increasing the risk of dangerous new variants.
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