How past COVID-19 infections may provide less protection against delta variant


AUSTIN (KXAN) — For months, Austin health officials have encouraged vaccinations as a means of building herd immunity against COVID-19.

Herd immunity is created through two scenarios: vaccination or the presence of antibodies from previous infections. However, these antibodies from previous COVID-19 infections might not stack up against the new delta variant, said Jeff Taylor, senior epidemiologist at Austin Public Health.

“The change may be great enough where the immune system says, ‘well, I’ve seen something similar in the past, but it’s not exactly the same,'” Taylor said. “So all the antibodies I produced in the past to the alpha variant may not protect me from this new variant.”

While there have been some reports of breakthrough cases — confirmed positive COVID-19 cases in vaccinated people — within the Austin-Travis County region, Taylor said the vast majority of these cases result in mild symptoms, as opposed to hospitalizations and deaths. He said the efficacy of the vaccine is still effective in reducing the severity of symptoms in fully vaccinated people.

“The vaccines still have protection in protecting those from severe infections and death. But clearly, it’s not 100% effective in preventing a reinfection,” he said. “And they don’t market them as being 100% effective and safe. We say 80% to 85% infected.”

Taylor later added: “That’s our effort to get people vaccinated — not only to prevent the cases, but to prevent those deaths.”

APH shared the latest updates on the delta variant in a joint meeting between Austin City Council and Travis County Commissioners Court Tuesday. In that discussion, APH medical officials noted the herd immunity threshold, or the percent of immune people to effectively mitigate the spread of a disease, has risen to 80% to 89% for the delta variant.

This number marks a substantial increase from previous herd immunity threshold figures, which estimated herd immunity emerging at around 70% of the population being immune.

Why a herd immunity threshold increases comes down to the contagiousness of a particular strain, Taylor said. The delta variant has a larger viral load than the alpha variant, meaning infected people carry more of the virus and therefore are more likely to spread it to a larger number of people.

“That strain has more viruses produced in those cells, and then people are spreading more virus when they cough, sneeze and talk,” he said. “And so that’s what may increase the herd immunity level, because people are more infectious with this virus, with the delta variant, than they were with what we say is the alpha variant.”

When it comes to the mortality rate of COVID-19, Taylor uses an analogy all Austinites are bound to be familiar with.

“The UT stadium holds just over 100,000 people, and the mortality rate for this virus has been around 1% overall. So, 1% of 100,000 people is 1,000 deaths,” Taylor explained. “So, if we can vaccinate 100,000 more people, we potentially will prevent 1,000 deaths. And so that’s what the current vaccine does, it prevents severe infections and deaths.”

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