New coronavirus cases are up 145% in the U.S. from where they were two weeks ago. COVID-19 hospitalizations are up 90% in the Texas Medical Center over that same time with roughly 100 new patients being hospitalized with COVID-19 each day across the system's hospitals.

All of this marks the beginning of a fourth wave of COVID-19 in Houston, one that can be attributed to the new delta variant of the virus that has become the dominant strain both in the region and the country, TMC officials said in a July 20 virtual webinar.

The key difference between this wave and those that preceded it is who is at risk, said Dr. Paul Klotman, president, CEO and executive dean of Baylor College of Medicine.

"What is going on now in our community is the people who are not vaccinated are totally susceptible, and it's a more infectious and more serious virus," Klotman said during the webinar. "The vast majority of people in the U.S. getting infected are folks who have not been vaccinated."

Although vaccinated people can still get the virus, with about 10%-15% of vaccinated people experiencing breakthrough cases, Klotman said they are far less likely to need to be hospitalized. About 99.5% of coronavirus deaths in recent weeks have been unvaccinated individuals, he said.

Although many questions remain about the delta variant and how it differs from other variants of the coronavirus, Dr. Peter Hotez, dean of Baylor's National School of Tropical Medicine, said research in China is beginning to show that people infected with the variant shed more virus. This would explain both why the virus is more transmissible as well as why there are more breakthrough cases for vaccinated individuals, he said.

As a result, Hotez said people may want to rethink use of masks in certain situations.

All the guidance by [the U.S. Centers for Disease Control and Prevention] about vaccinated people not needing masks were based on original lineage, which made a lot of sense," Hotez said. "If it’s true the delta is being shed in so much more abundance by unvaccinated and even vaccinated individuals, we may need to think about revisiting that. Vaccinated people may want to think about wearing masks indoors in areas of high transmission."

Also at play is the part of the country a person lives in, Hotez said, and what percentage of the population is vaccinated in that region. Places like Massachusetts and Vermont, where almost all adults and most adolescents are vaccinated, may be fine continuing to follow CDC guidelines as they are, Hotez said.

Differences are also seen within Texas, Hotez said, where higher vaccination rates are seen in areas along the Mexico border and in large urban areas like Houston, and lower vaccinations rates are seen in rural parts of east Texas and in the panhandle.

"I think there is going to be a real vulnerability [in East Texas] ... that could start to look like Missouri or Arkansas over the next few weeks," Hotez said, referencing parts of the U.S. where hospitalizations have been on the rise.

As the demographics of people getting infected with the coronavirus shift toward a younger population, both Klotman and Hotez emphasized that young people are not invulnerable. With the new school year starting Aug. 23 for many districts, Klotman said he believes there should be a requirement for all adults in schools to be vaccinated and for all children to wear masks.

The social disruption of the past year also led to delays in children getting other vaccinations, such as measles, which Hotez said could come into play this year.

"I’m holding my breath as kids get back to school about a potential measles outbreak like we saw in 2019," he said.

Despite the concerns, the fourth wave of the coronavirus is not expected to be as sharp as previous waves, said Klotman, adding that hospital capacity will not become overwhelmed in Houston.

"We saw three very big peaks in the past. This one will be more of a prolonged slow rise and will peak at much less than the peaks we had before," he said. "But this virus is very good at finding unvaccinated people, and we’ve only vaccinated half of the country and half of our region in Houston."