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As the more contagious delta variant circulates, public health experts say herd immunity has become an even more distant goal

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Jaimy Lee

The delta variant is twice as contagious as the original strain of the coronavirus that causes the disease COVID-19, and its ability to infect so many more people, including a small number of the fully vaccinated, is going to make it even harder to achieve herd immunity.

“Delta is here, vaccinations have stalled, and breakthrough infections are happening,” Dr. Ricardo Franco, an associate professor of infectious diseases at the University of Alabama at Birmingham, said Tuesday at an Infectious Diseases Society of America briefing on the definition of herd immunity.

Getting infected with delta means that both vaccinated and unvaccinated people carry higher viral loads compared with other strains of the virus. If it’s easier for the virus to spread, that means the threshold for herd immunity is even higher than it had been before, experts say.

Up to 85% or even 90% of the population will likely need to be immune to the virus to achieve herd immunity, compared with estimates of 50% , 60% or 70% to 85% for earlier strains of the SARS-CoV-2 virus. “For delta, threshold estimates are well over 80%, maybe approaching 90%,” Franco said.  

“The more infectious the strain, the higher the number,” Dr. Gabor Kelen, who chairs the department of emergency medicine at Johns Hopkins University, said in an interview last week. (He predicts the new threshold is at least 85%.)

Experts say that a higher threshold may mean we’ll need a higher rate of immunization, and that is going to create a challenge for a country that has struggled to get shots in arms. Although vaccinations are ticking up of late, they have largely remained flat for weeks, and only 58.2% of qualified people in the U.S. — those who are 12 years old or older — were fully vaccinated as of Wednesday.

“The higher the threshold, the better your vaccination campaign has to be,” Franco said in an interview. 

Here’s how herd immunity works:

Herd immunity is an epidemiological concept, and not a “mathematical certainty,” as described by Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases and chief medical adviser to President Joe Biden, back in June . It evaluates how many people need to be immune to a disease to establish a baseline of protection for the community as a whole. At least 95% of the population has to be vaccinated against the measles to prevent outbreaks, for example. 

Herd immunity, which takes into account a combination of vaccine-induced protection and natural immunity from infection, is calculated based on infection rate. It uses the basic reproduction number, which is often referred to as the R0. 

The original strain of the virus has a R0 of 2.5, and the alpha variant, which was first identified in the U.K., has an R0 between 4 and 5, Franco said. Delta’s R0 is thought to be between 5 and 8.

“Because this [strain] is more infectious, it just means you have to achieve a higher level of immunity to battle it,” said Jennifer Kates, director of global health and HIV policy at the Kaiser Family Foundation. 

That means one person infected with delta could transmit the virus to five to eight other people if there are no other mitigation measures such as masks or vaccination in place.

In addition, new research indicates that some people who are fully vaccinated can get infected with delta. A study published Friday found that 74% of the people who tested positive for COVID-19 in a recent outbreak in Massachusetts had been fully vaccinated. 

“The delta variant may be transmissible to a smaller degree, but to some degree, in people who are vaccinated,” RBC Capital Markets analyst Brian Abrahams told MarketWatch this week. “So the previous idea that with 60% or 65% of the population exposed to the virus or having been vaccinated, that would be the threshold — that’s probably not exactly the case anymore.”

That said, there are other factors to consider. We don’t know how many people have been infected with the virus. At the outset of the pandemic it took months to get testing up and running. Not everyone gets tested. About 40% of people who contract the virus are thought to be asymptomatic; they may only get tested as part of workforce or school surveillance or if they are exposed to someone who tests positive. The results from convenient at-home COVID-19 tests aren’t tracked. 

“We may be closer to that [threshold] than the case numbers might actually predict,” Abrahams added.

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