AUSTIN (KXAN) — With 29 confirmed cases of the COVID-19 delta variant in Austin-Travis County as of August 3, Austin Public Health officials broke down just exactly how contagious the variant is, and what percentage of the population needs to be vaccinated to achieve herd immunity.

Dr. Desmar Walkes, Austin-Travis County health authority and medical director, presented the latest findings during a joint session between Austin City Council and Travis County Commissioners Court Tuesday. Walkes said the region’s seven-day moving average has entered Stage 5 levels.

“The science has changed,” she said. “We have a new variant, the delta variant, and it’s impacted our hospital systems and our medical systems because it spreads much more rapidly.”

An additional 76 people were admitted to hospitals Tuesday, with a total admissions count of 433, including 153 patients in intensive care units.

Walkes said the delta variant has “100 times the viral load” in patients currently contracting the disease. But what exactly does that mean?

Decoding transmission thresholds

Walkes outlined the new science that suggests a higher percentage of people need to be vaccinated in order to reach herd immunity.

Previously, Austin-Travis County medical officials said 70% vaccination rates would help bolster herd immunity for the region. However, that’s no longer the case with the delta variant, Walkes said.

“The science has changed because this virus, just like all living creatures, has found a way to survive by mutating itself,” she said. “It’s spreading a lot easier in our community, and so it’s going to require that we have higher herd immunity thresholds.”

Under the COVID-19 alpha — or United Kingdom — variant, approximately 75% to 80% of people would need to be vaccinated to reach herd immunity. Comparatively, the delta variant is significantly more contagious, and ultimately requires 80% to 89% of individuals become immune in order to reach that herd immunity status.

The delta variant is not only more contagious than other COVID-19 strains, but well known infectious diseases, as well.

The delta variant is nearly as contagious as chicken pox, a disease whose herd immunity is dependent on 90% to 92% of community members being immune. Whooping cough, another respiratory disease, has an 82% herd immunity threshold — comparable to the UK variant, but less contagious than the delta variant.

The delta variant has nearly four times the herd immunity requirements of influenza, or the seasonal flu. The flu only requires immunity among 23% of community members in order to establish herd immunity.

Why is herd immunity important?

When a population has reached herd immunity, it minimizes the risk of an infectious disease spreading at a higher rate of transmission. When a significant portion of the community becomes immune, the entire community — those who are and aren’t immune alike — are protected.

“Often, a percentage of the population must be capable of getting a disease in order for it to spread. This is called a threshold proportion,” reads a June 2021 Mayo Clinic analysis. “If the proportion of the population that is immune to the disease is greater than this threshold, the spread of the disease will decline. This is known as the herd immunity threshold.”

More than 447,000 doses of COVID-19 vaccines have been distributed for eligible residents (those ages 12 and older) within Austin-Travis County thus far. Statewide, the Johns Hopkins Coronavirus Resource Center reports more than 27.5 million vaccine doses have been administered to eligible Texans, with 45.2% of the state’s population fully vaccinated.

Dr. Park Hudson of UT Dell Medical School said the delta variant mirrors symptoms of traditional COVID-19 strains, but spreads at a much higher transmission rate than makes unvaccinated people all the more susceptible. Hudson said the delta variant is two to four times as contagious as previous strains.

“This variant is so contagious, it will find groups of unvaccinated people. So really, now is the time to get vaccinated,” he said in a UT Dell Medical COVID-19 explainer video.

“Vaccines are highly effective at reducing severe disease and death, including from the delta variant.”

Previous research on COVID-19 reported its contagiousness was on par with the flu or common cold, said Dr. Jan Patterson, an infectious disease expert with UT San Antonio and a member of the Texas Medical Association’s COVID-19 task force.

Now? The pervasiveness of the delta variant has revealed new information on just how contagious it is.

“What we’ve seen with this COVID is that initially it was about as contagious as the common cold, and about the same as flu, and that was in the range of like, say one person might make two people or sometimes three people sick,” she told KXAN in a July 30 interview. “With this new delta variant, what we’re seeing and what’s estimated is that one person can make eight to nine people sick.”