[Image courtesy of CDC]

Delta is “an extremely transmissible variant and has taken us all for a loop,” said Dr. Monica Gandhi, a professor of medicine and associate division chief of the division of HIV, infectious diseases and global medicine at University of California, San Francisco.

But despite Delta’s hurricane-like ascent, the volume of infections it drives are likely to fall just as quickly after peaking, Gandhi said in a webinar from the Bay Area Council.

Delta could peak anywhere from mid-August to mid-October, according to several experts quoted in WebMD.

In the interim, Delta will remain most threatening for the unvaccinated and those without prior COVID-19 infections, although the variant will likely continue to drive breakthrough infections in vaccinated individuals.

In highly vaccinated areas of the U.S. where the vaccine has spread, there has been a “decoupling” between infections and hospitalizations, Gandhi said.

That decoupling is a function of protection from T and B cells. Much of the conversation about COVID-19 has focused on antibody levels because they are generally straightforward to measure. But B and T cells are two major pillars that the immune system uses to protect against future infection.

“We don’t talk enough about the T-cell immunity that is generated by all of these vaccines,” Gandhi said. A strong T-cell response protects against severe disease in the long run. For example, the T-cell response in vaccinated people has played a role in containing hospitalization rates in the U.S. as the Delta variant spreads. Conversely, in areas with low vaccination levels, such as Arkansas and Florida, hospitalization rates have exploded, resembling prior outbreaks.

In addition to T cells, immunological memory relies on memory B cells, which offer a “blueprint to make new antibodies,” Gandhi explained.

In vaccinated individuals, B cells stimulate the production of antibodies after exposure to SARS-CoV-2. “It may take a couple of days, which is why we are seeing more mild breakthrough infections with the Delta variant,” Gandhi said. Nevertheless, Delta’s ability to cause mild breakthrough infections supports a return to mask-wearing for vaccinated individuals.

It is normal for antibody levels to wane over time. “If we kept all the antibodies in our bloodstream that we’ve seen from every infection and vaccine, our blood would be as thick as paste,” Gandhi said.

What is notable about SARS-CoV-2 is its potential to cause severe disease. And vaccines “absolutely do protect us” from severe and critical COVID-19 cases.

One wrinkle is that Delta is associated with higher viral loads in vaccinated individuals than prior variants. Vaccinated individuals infected with the Delta variant are likely to spread the virus but are less likely to transmit it than unvaccinated people exposed to it. Before the Delta variant, several studies indicated that vaccination dramatically reduced asymptomatic infection. “Unfortunately, the Delta variant has changed that equation,” Gandhi said.

When gauging the likely future impact of Delta, the experience in countries such as India, the U.K. and Israel provide clues. The U.K. is about six to seven weeks ahead of the U.S in dealing with the variant. Case counts there began plunging around July 24. Cases dropped similarly in India, even though it has a lower vaccination rate.

Relying on the U.K. experience to extrapolate the likely trajectory of the Delta wave, cases are likely to peak sometime in August and go down in the U.S. by mid to late September, Gandhi said.

Ultimately, Delta could be the peak of transmissibility for SARS-CoV-2.

While some experts worry that SARS-CoV-2 is “a few mutations away” from evading vaccines, future variants are unlikely to do so, Gandhi said. Immunity from T cells and B cells provides resilience.

Scientists have projected that SARS-CoV-2 would evolve to become more transmissible. “But it’s actually not smart for a virus to become more deadly because it kills its host,” Gandhi said.

But it is not in a virus’s best interest to mutate excessively. “There are fitness costs for too much mutation,” Gandhi said.