A leading US disease expert says there's 'no doubt in my mind' that vaccinated people are helping spread Delta

• Vaccinated people are well protected from severe illness and death even as the Delta variant surges.
• But it's possible for fully vaccinated people to be asymptomatic and spread COVID-19 to others.
• A top disease modeler who advises the White House said vaccinated people should still wear masks.
• See more stories on Insider's business page .

The US is celebrating robust COVID-19 vaccine coverage.

Strangers are standing shoulder to shoulder in bars , fans are singing along at packed indoor concerts , and travelers are flying in numbers not seen since before lockdowns began in 2020.

"While the virus hasn't been vanquished, we know this: It no longer controls our lives," President Joe Biden said on Sunday, as hospitalizations, cases, and deaths trended down. "America is coming back together," he added.

But a quiet new wave of severe COVID-19 infections is brewing, fueled by the more transmissible Delta coronavirus variant.

"We actually have states where hospitalizations are going up more than cases," Christopher Murray, the director of the Institute for Health Metrics and Evaluation, told Insider, stressing that data from the Centers for Disease Control and Prevention may mask the virus' true spread.

As the CDC's guidance is not to test vaccinated people unless they're symptomatic , "we're probably missing a bunch of transmission in vaccinated individuals," Murray said.

Delta is spreading quickly in the US

Drilling into state-level data reveals how quickly Delta has spread .

"We have 14 states where transmission has started to go back up," said Murray, who's also the lead modeler at the IHME, which the White House has leaned on for disease projections throughout the pandemic.

That's "due to the Delta variant and the fact that everybody's stopped wearing a mask and just basically stopped most precautions," he added.

Disease modelers at Scripps have estimated that Delta could be responsible for about 60% of COVID-19 cases across the US.

Vaccines prevent serious illness

COVID-19 vaccines don't prevent every infection - they are designed to better defend your body against the virus. The vaccines authorized in the US do that very well, even against Delta.

Some vaccinated people get a mild, cold-like illness , with a headache and a runny nose. Others could get infected but never know it, becoming silent spreaders.

Delta has wreaked far greater havoc among the unvaccinated. Hospitalizations are trending up in several states, including Missouri, Arkansas, Utah, and Mississippi, according to IHME data . Those are some of the same places where vaccination rates are lagging .

How Delta can move through a semivaccinated population

Tim Spector, an epidemiologist at King's College London, previously told Insider that while there's no evidence that Delta is deadlier, it is more infectious, and "because of that extra stickiness, it's going to still keep breaking through the vaccine group."

More than half of Scotland is fully vaccinated, and 71% of Scots have received at least one dose of a vaccine. But the country is suffering its worst wave of infections.

"You cannot explain the explosive epidemic in Scotland, in a pretty highly vaccinated population, if they're not playing a role in transmission," Murray said of vaccinated people.

However, Will Lee, the vice president of science at Helix, a testing company helping the CDC track variants , said areas with higher vaccination rates tend to have fewer cases.

Lee pointed to studies indicating that Delta cases are milder in vaccinated people and, therefore, people are not infectious for as long. It stands to reason, he said, that vaccinated people would not transmit as much.

"That window of transmission probably goes down," he said.

Delta versus our vaccines

A recent real-world study from the UK suggested that Pfizer's vaccine was about 88% effective in preventing symptomatic COVID-19 with Delta, markedly lower than the 95% efficacy against earlier-detected strains.

Vaccines from Moderna and Johnson & Johnson , too, may be less effective at preventing symptomatic infections with Delta, early lab studies by those companies have suggested.

What's clear is that all three US-authorized vaccines maintain strong protection against severe disease and death, even with the Delta variant.

While natural immunity may help (federal estimates suggest that more than one-third of Americans have had COVID-19), Russia is an example of how prior infections can't halt Delta's spread.

Masks work

Murray says COVID-19 outbreaks are being investigated in US groups "that are 90%-plus vaccinated."

"That could only be occurring if they're transmitting amongst each other," he said. "There's no doubt in my mind."

That's one reason many infectious-disease experts still wear face masks indoors .

"In our models, we see that even modest mask use combined with vaccination can really put the brakes on even the Delta variant," Murray said.