Delta has set back herd immunity estimates. Heres how that affects the course of the pandemic – The Dallas Morning News

A bag containing COVID-19 vaccines rests on a table during the last day of vaccinations at the drive-thru Fair Park site on Saturday, July 17, 2021, in Dallas.

When health officials announced in early July that 80% of Dallas County residents were immune to the coronavirus, it was cause for celebration. Cautious celebration.

At the time, health officials agreed that if 80% of the population were immune to COVID-19, people could expect herd immunity the condition in which people are indirectly protected from infection because widespread vaccinations and previous exposure limit a diseases spread.

But with the rapid transmission of the delta variant, a more contagious strain of COVID-19 that federal health officials estimate accounts for a little more than 94% of all cases in the United States, the expectation for herd immunity has changed.

But health experts say herd immunity can still be achieved through vaccinations. Heres what you need to know about the moving target of immunity estimates.

In Dallas County, the Parkland Center for Clinical Innovation tracks progress toward herd immunity.

Before the delta variant began spreading, the center estimated the proportion of people who would need to be immune to the virus to prevent infections from spreading widely by adding the number of vaccinated people to the number of people who have natural immunity after recovering from the virus.

Then the team added the estimated number of people who had contracted the virus but never got tested for it.

However, the delta variant has made that third step more challenging.

Data on the 2020 COVID-19 strain indicated people who tested positive for the virus had some protection after recovering, possibly as long as six months, the center previously said.

But health experts say theres not enough information about how people with natural immunity are protected from the delta variant or how people are protected after recovering from it.

Ive had patients that had COVID previously and are now coming down with delta, said Dr. John Flores, chairman of the North Texas Medical Society Coalition, which represents physicians in five North Texas counties. Now, most of these patients werent vaccinated in the first place. But we are seeing that a previous COVID infection does not protect you against the mutated strain.

Steve Miff, president and CEO of the Parkland Center for Clinical Innovation, said that as of Aug. 10, about 41.5% of Dallas County residents were fully vaccinated, and another 12.3% had tested positive and recovered. The remaining group, which Miff said represents about 39% of the population, is no longer being counted toward herd immunity progress.

You have this group of 40% of the population who likely have been impacted but were asymptomatic or their symptoms were so mild that any antibodies that they built, its unclear, but its very likely that their protection is not strong enough for delta, he said.

Health experts generally do agree, however, that vaccines provide far better protection than natural immunity does.

The center is still working to come up with an estimate for herd immunity in the wake of the delta variant, but Miff said the number is likely to be as high as 88% to 90%.

Dr. Philip Huang, director of Dallas Countys health department, said its important for people to remember that reaching herd immunity was never considered the finish line.

There is not a magic number with this, and the game keeps changing, he said.

Health experts say its important to note that herd immunity depends not only on the level of immunity in a community but also on how infectious a disease is. Variants and the evolving pandemic make herd immunity a moving target.

In the absence of any preventive measures, the 2020 strain of COVID-19 was estimated to have a reproduction number of two to three, said Dr. Catherine Troisi, an infectious-disease epidemiologist at the UTHealth School of Public Health.

The number, which is also referred to as R0 (pronounced R naught), represents how many new cases are expected to result from one case.

Troisi said epidemiologists estimate that the delta variant has a reproduction number between six and eight.

Lets say its seven. Number one, your index case, infects seven people, she said. Each of those seven goes on to infect seven more. Now youve got 49, 50, 51, 52 people infected, and you can see how it just spreads like wildfire.

And the more effectively a disease spreads, the more people need to be protected to stamp it out.

Also, though COVID-19 vaccines are incredibly effective at preventing severe symptoms and death from the coronavirus, they dont prevent people from spreading COVID-19 to others.

Even if 100% [of people] were vaccinated, it might still be possible for some transmission to occur, Troisi said.

Some health experts, including former FDA Commissioner Scott Gottlieb and a former official under President Joe Biden, have theorized that cases will rise for several weeks and then begin dropping off.

That idea is based on data from the United Kingdom, where the delta variant fueled a sharp rise in cases between June 1 and mid-July. Cases started to decline about July 17, according to government data.

A similar trend has been seen in India, where cases spiked between March and early May before dropping off, according to data from Reuters.

Miff said the PCCI team expects three to four more weeks of growth before cases drop if the United States follows those patterns.

But other experts are wary of counting on the delta variant to behave the same way in the United States, as its not known for certain why cases dropped in the United Kingdom and India.

While the vaccination rate in the United Kingdom is higher than in the U.S., India lags far behind both places. Almost 60% of people in the U.K. are fully vaccinated, compared with about 51% in the U.S., according to Reuters. In India, only 8.5% of people are fully vaccinated.

Theres a lot of people in India, and the vaccination rate is very, very low, so basically everybody is, or was, susceptible. I would be surprised if it burned itself out so quickly, Troisi said.

Its also problematic to rely on the virus to run its course because of the stress doing so would put on health care systems and unvaccinated people, Flores said.

If thats the case, were going to have other problems besides worrying about where delta is, he said. Not only that, but you have to consider the long-term consequences of COVID. ... Ive had patients that have had kidney failure, cardiomyopathies, blood clots and those are problems that can persist.

Huang said for now, health officials are still concerned by the exponential case growth in Dallas County and cant predict when cases may drop off.

What were really worried about is these numbers are going to continue to grow, Huang said. It may be a shorter peak, so it may hit a really high number, it may hit higher numbers in the hospitals than weve ever seen, and that may be for a short period of time potentially, but we dont know.

Either way, health experts say the data is clear: Vaccination is the best way to curb the rise in cases.

As infections have surged, upticks in vaccine uptake have been seen across the country, as well as locally. For a while in Dallas County, only about 15,000 people were getting a shot each week, Miff said. That number went up to 25,000 and then to 35,000 over the last two weeks, he said.

Theres strong indications that protection continues to be strong, he said. Thats why focusing on the vaccines is the most tangible thing that we can do.

But without a continuing rise in inoculations, the virus will have more opportunities to mutate, potentially becoming stronger and causing more severe illness.

With so many unvaccinated people, every single person is kind of like a little laboratory for the vaccine to mutate, Flores said. When that gets into your body, you reproduce it and it does things a little bit differently when it mutates. And so if theres a favorable new virus that can spread faster, its going to take over. Thats what delta did.

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Delta has set back herd immunity estimates. Heres how that affects the course of the pandemic - The Dallas Morning News