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Doctors say herd immunity from COVID-19 is unlikely, despite El Paso mayor's claim

Because the delta variant is so infectious, doctors say herd immunity may be mathematically impossible.

SAN ANTONIO — El Paso Mayor Oscar Leeser this week said his city is the first in Texas to earn herd immunity from COVID-19, but doctors question whether herd immunity is still an achievable goal.

Herd immunity happens when enough people become immune to a virus that even the unvaccinated or previously uninfected are protected. In theory, achieving herd immunity is the first step toward eradication of a virus.

Herd immunity is based on an equation that accounts for how fast a virus is spreading. As viruses mutate, the math changes.

Leeser's claim is based on outdated numbers that assume the virus spreads slower than it does today. A little more than 75% of eligible El Paso residents are vaccinated, which might have eclipsed the herd immunity threshold before the more infectious delta variant was dominant.

"Early on, you probably could have gotten to 75 or 80% vaccine rates and stopped the spread," said Dr. J.B. Cantey, an associate professor of pediatric infectious disease at UT Health San Antonio. "Unfortunately, the delta variant is a lot more contagious."

Leeser's math does not account for children, either. Even using the dated formula, herd immunity would require 75% of all residents to be immune, not 75% of the vaccine-eligible.

It's not clear how long a previously infected person is protected from re-infection, which makes it difficult to account for their contribution to community immunity. Doctors believe the vaccine confers greater viral protection than a prior infection.

But because vaccines are not 100% effective, there is some concern that herd immunity may be mathematically impossible. Breakthrough infections are not common, but they are possible.

"That's why some people have started to say, 'Hey, with the delta variant, even if everyone got vaccinated, we might not be able to reach the finish line,'" Cantey said.

Centers for Disease Control data indicates a person with the delta variant will infect between six and nine other people, on average.

"On paper, using that math equation, it is possible that herd immunity might be beyond us right now," he continued. "But it's still extremely important for everyone who can to go get vaccinated."

The hope, Cantey says, is that higher vaccination rates would force the virus to mutate into a weaker variant that more closely resembles the common cold.

"One of these variants will become the new dominant, circulating virus," Cantey said. "Maybe that strain is better, not as aggressive, not as contagious. All of a sudden, herd immunity is possible again."

"We've got to ignore headlines that say, 'We can't reach herd immunity' or, 'We have reached herd immunity,'" he continued. "We've just got to put our heads down and get shots in arms. That will get us through this."