On Wednesday, news broke that Dallas County had reached the 80% herd immunity threshold. To hit that milestone, the Parkland Center for Clinical Innovation (PCCI) reported that 46.6% of the county’s population had been vaccinated and 48.7% had become naturally immune after recovering from the disease. (PCCI is a separate entity from the Parkland hospital system.)
Some say that pronouncement is premature.
“I’m really shocked that there’s even a statement made that we have herd immunity. We don’t. It’s just a scientific fact,” said Dr. Erin Carlson, an associate clinical professor in the College of Nursing and Health Innovation at the University of Texas at Arlington.
In certain cases, herd immunity is attained when around 70% of a population has become protected from a disease via vaccination or past infection, Carlson said. But COVID-19 doesn’t fit into that definition: Only the vaccinated should be included in such calculations.
It's unclear how long protection from natural immunity lasts, she said. Some who had recovered from the coronavirus later became re-infected, meaning they could also be carriers once more.
"With COVID, we don't have the science yet to know enough about … the antibodies that come from natural infection," Carlson said.
At the pandemic’s outset, the nation’s top infectious disease experts warned the country wouldn’t reach herd immunity until 60% – 70% of the population had been vaccinated, she said. But over time, the goal continued to creep higher as new, more infectious variants emerged.
Delta is the latest variant ravaging the U.S., and Carlson said it spreads faster than previous versions. There are also early indications that it may result in more severe illness.
The Delta variant has been linked to outbreaks in the Houston area and is now the most common strain detected in one of that city’s hospital systems, according to Houston Public Media. It has also begun to spread in Dallas County, which on Wednesday reported a four-day total of 319 additional coronavirus cases and two deaths.
With the initial coronavirus variant, one person with COVID-19 could infect up to 2.5 others, Carlson said. Later, the B.1.1.7. variant pushed the number of transmissions to around four, and then the Delta variant doubled it to about eight.
Less than half of Dallas County’s population has been fully vaccinated, leaving a lot of room for initial infection or reinfection, Carlson said. The numbers don’t look great for the country either.
“This is still raging everywhere else. And as long as it’s raging everywhere else, it can rage here, too." – Dr. Erin Carlson
“The fact that 30% of the United States population has said they will not be vaccinated is frightening,” she said. “That means we cannot reach herd immunity nationally. If we cannot reach herd immunity nationally, we cannot reach it locally.”
Even if North Texas does reach herd immunity, it won’t mean anything until it happens worldwide, Carlson said. Diseases don’t know to stop at borders. Anyone who's unvaccinated is still at risk of catching the disease.
As long as there are transmissions, there will be mutations, and as long as there are mutations, there will be new variants, she said. It only takes “one little tiny piece of that genome to be miscopied” for a new variant to emerge that can outflank the vaccine.
Dallas residents should continue to keep their masks handy, Carlson said. The pandemic is far from over.
"This is still raging everywhere else," she said. "And as long as it’s raging everywhere else, it can rage here, too."