The Delta Variant: What it Means for the Vaccinated and Unvaccinated

Recent breakthrough infections have experts looking closely at the true risks of the COVID-19 delta variant and how we can protect ourselves.

Less than 2 months ago, President Joe Biden evoked a “summer of freedom” from COVID-19 during a speech urging people to get vaccinated. For a little while, at least, the country seemed to be on the precipice of ending the pandemic.

Fast forward to early August, and the vicious delta variant wreaks havoc as it swirls across the nation and the globe. Cases, hospitalizations, and deaths have risen in areas with lower-than-average vaccination rates, alarming epidemiologists and healthcare workers who recall previous surges all too well.

What does this trend look like on the ground? “In our 1100-bed university hospital, we have seen an increase in hospitalizations every week since the end of June,” said Ricardo Franco, MD, assistant professor of medicine and associate scientist in the Center for AIDS Research at the University of Alabama at Birmingham, speaking at a media briefing hosted by the Infectious Diseases Society of America (IDSA). “It’s now over 60 active cases in our COVID-19 units, from as low as 3 active cases observed in mid-June.” Unvaccinated patients account for 97% of COVID-19 admissions, he said, and roughly 89% of the samples sequenced at the hospital laboratory contain the delta variant.

Alabama currently has a test positivity rate of 22%, according to Franco, who said he has seen modeling estimates projecting double the number of infections that the state experienced this past January. Because the delta variant is twice as transmissible as the original strain of COVID-19, the threshold for herd immunity is pushed higher. While the herd immunity threshold for original COVID-19 was about 67%, the threshold estimates for the delta variant approach 90%.

The news is not all bad, however. In areas of high vaccination, unvaccinated people benefit from the greater herd immunity protection bestowed by the vaccinated people around them, Franco pointed out. And as vaccination rates tick up nationwide, this protection should only increase.

To get to the greatest level of protection will require changes on everyone’s part, said Emanuel Ezekiel, MD, PhD, the vice provost for global initiatives, Diane v.S. Levy and Robert M. Levy University professor, and co-director of the Healthcare Transformation Institute at the University of Pennsylvania, who also spoke at the briefing. Chief among these changes is consistent mask use among the vaccinated and unvaccinated alike. Ezekiel recommends the use of high-quality masks, such as N95s, in indoor and some outdoor settings.

Asked about the feasibility of big summer gatherings, Emanuel expressed concern: “Even though it’s outdoors and there’s a lot of air circulation, large crowds where people are close together for prolonged periods of time is a problem. While you might not have high transmission in those situations, you have a very high[ly] transmissible virus in the area.” The mobility of attendees also poses a problem, he noted: “People are traveling and then they’re going to go back to [their] communities, potentially seeding lots of other communities.”

While breakthrough infections are still considered rare, it's difficult to determine their true incidence because testing rates have fallen. “If you go to many places, it’s actually hard to get a test and hard to figure out where to get a test,” Emanuel said. “[A] lot of [breakthroughs] are asymptomatic, and so we really don’t have a good handle on the frequency of asymptomatic but nevertheless apparently transmissible infections with delta.”

Not only does the dearth of testing mean we may be missing many COVID-19 infections, it also means infections are not being genotyped. Without this information, it’s harder to monitor disease trends and keep tabs on new variants that may be brewing.

Does past COVID-19 infection protect against the delta variant and contribute to herd immunity? One thing is clear: There are no definitive answers yet. While studies indicate that past infection provides some measure of protection, they were conducted before the more transmissible delta variant became widespread.

“We simply don’t have enough information right now to know whether natural immunity is going to provide a protective effect against delta, but it is assumed that it would not,” said IDSA president Barbara Alexander, MD, FIDSA, professor of medicine and pathology and director of Transplant Infectious Diseases Service at Duke University School of Medicine.