At the start of the delta surge in mid-July, nine out of 10 local hospitalizations for COVID-19 were unvaccinated people. Over the past two months, that ratio has dipped as low as eight in 10.

So are more vaccinated individuals contracting COVID-19 and ending up in the hospital?

Local health officials say the answer isn’t as simple as yes or no, and that San Antonians should definitely still get the jab. For certain Pfizer vaccine recipients, that includes the newly approved booster shot as well.

Yes, as the pandemic lingers, health officials are expecting these “breakthrough” cases to rise, said Anita Kurian, the assistant director of Metro Health, during a briefing Tuesday.

But rising vaccinations rates, coupled with declining cases overall, tell a more nuanced story, said Cherise Rohr-Allegrini, a local epidemiologist and former research director of the Immunization Partnership.

The combination of more vaccinations and fewer cases gives the appearance that more vaccinated people are getting breakthrough infections and being hospitalized, Rohr-Allegrini said.

“When the majority of the population is unvaccinated, the majority of hospitalized cases will be those who are unvaccinated, because they make up most of the denominator,” Rohr-Allegrini told the San Antonio Report via email. “As more people are vaccinated, that denominator changes. … [it] is getting smaller, and the proportion of immunized cases will appear larger, even if it’s the same number.”

At its highest point in early August, the delta surge pushed San Antonio’s positivity rate up to 21.4%, meaning about one out of five COVID-19 tests were positive. Over the past month and a half, that percentage has dropped back down to 5%, meaning one out of 20 tests is positive.

Meanwhile, at the start of the delta surge about 58% of Bexar County’s eligible residents were fully vaccinated and 73% were partially vaccinated. As of Tuesday, 73% of Bexar County eligible residents are fully vaccinated, whereas 89% are partially vaccinated, according to city data.

The vaccine was never promised to be 100% effective, Rohr-Allegrini added. No vaccine is perfect and the COVID-19 vaccines are no exception, she said. If the COVID-19 vaccines are about 85% effective, 15% of those vaccinated won’t develop sufficient immunity to prevent infection.

So it’s not that more vaccinated people are getting sick, Rohr-Allegrini said, but that of those who are sick, a greater proportion are vaccinated because more people are vaccinated and the vaccine isn’t perfect.

Dr. Bryan Alsip, executive vice president and chief medical officer at University Health, agreed with Rohr-Allegrini and said breakthrough cases that are serious enough to require hospitalization are mostly happening among older individuals and those whose immune systems are compromised.

It’s those two categories of patients that led the Food and Drug Administration and the Centers for Disease Control and Prevention to recommend a third dose, Alsip said.

People in those categories “may have not mounted a sufficient enough immune response to equate what others have,” he said. “So [the agencies] recommended a third dose to help them.”

Metro Health is offering booster shots of the Pfizer vaccine to those 65 and older, as well as adults 18 and older considered at high risk of severe illness from COVID-19, starting Wednesday at the Alamodome from noon to 8 p.m. Wednesday through Friday.

Those eligible for the booster shot should bring their vaccination card to prove they have received both initial doses of the Pfizer vaccine, Kurian said. University Health is also administering Pfizer booster shots.

Ernesto Sarabia, center, receives his COVID-19 vaccine booster at Wonderland of the America’s on Friday, September 24, 2021.
Ernesto Sarabia, center, receives his COVID-19 vaccine booster at Wonderland of the America’s on Friday, September 24, 2021. Credit: Scott Ball / San Antonio Report

San Antonio’s declining positivity rate means that overall, the city is “moving in the right direction,” said Bernard Arulanandam, an immunologist in the University of Texas at San Antonio’s department of molecular microbiology and immunology and the university’s vice president for research in economic development and knowledge enterprise. Arulanandam encouraged unvaccinated people to go get vaccinated, and continue to mask up in close quarters.

People also shouldn’t be discouraged by the need for boosters — a lot of vaccines need boosters over time, including hepatitis, tetanus, flu, and others, Arulanandam noted.

“Even if we have breakthrough infections in some of the vaccinated, these vaccines are meant to protect from severe disease, and they do that really well,” Arulanandam said. “The vaccine is still efficacious.”

This story has been updated to correct the spelling of Dr. Bryan Alsip’s name.

Avatar photo

Lindsey Carnett

Lindsey Carnett covers the environment, science and utilities for the San Antonio Report.