Vaccinated can spread Delta, data says,

New U.S. data suggests fully vaccinated people can still spread variant

Alarming new COVID-19 data from the U.S. suggests that vaccinated people with breakthrough infections can spread the Delta variant as easily as those who are unvaccinated — data the head of Ontario’s science table called a “gamechanger” that underscores the need for continued indoor masking and higher vaccination targets than what policymakers are currently aiming for.

The data comes from a new paper by the U.S. Centers for Disease Control and Prevention that the agency published in its Morbidity and Mortality Weekly Report (MMWR).

It was also referenced in a leaked CDC presentation, obtained and posted online by the Washington Post, urging health officials to start acknowledging “the war has changed” thanks to Delta, a strain even more transmissible than the viruses causing Ebola, the common cold or chickenpox.

In its new MMWR study, the CDC described an investigation into a massive Delta-driven outbreak in Barnstable County, Mass., which infected 469 people with COVID-19, three quarters of whom were fully vaccinated. Seventy-nine per cent of vaccinated cases were symptomatic; worryingly, viral loads in many fully-vaccinated people were “similar” to those who had never been vaccinated.

These new data were “pivotal” in the CDC’s decision to reverse its position on masking on Tuesday, when it issued guidelines for vaccinated people to resume indoor masking in areas with high infection rates.

“High viral loads suggest an increased risk of transmission and raised concern that, unlike with other variants, vaccinated people infected with Delta can transmit the virus,” CDC director Dr. Rochelle Walensky said in a statement on Friday.

For Dr. Peter Juni, scientific director of Ontario’s COVID-19 Science Advisory Table, the new CDC data upends prior assumptions that vaccinated people are unlikely to spread the virus.

The implications of this, he said, are “massive,” particularly in jurisdictions such as Alberta where policy-makers are largely relaxing COVID-19 control measures and referring to the pandemic in past tense.

“This data is the nail in the coffin for any suggestion that we can go back to normal and drop masks in the foreseeable future,” Juni said. “There is no

way during the next few months that we can actually keep this pandemic under control without public health measures.

“We should not be naive — and especially not as naive as Alberta — in believing that this is over.”

In the leaked CDC presentation, which the Washington Post posted online Thursday, the public health agency noted that Delta is more transmissible, reduces vaccine effectiveness against infection, and may cause more severe disease.

The document cited research by Canadian scientists, including a preprint by University of Toronto epidemiologists David Fisman and Ashleigh Tuite, showing higher odds of hospitalization and death linked to the Delta variant.

For Tuite, the findings from the CDC presentation come as no surprise and delivers a message consistent with the one she and others have been conveying for months: that Delta is a formidable new threat, and the pandemic is far from over.

On Friday, public health officials presented new modelling data suggesting that Canada is now “at the start of the Deltadriven fourth wave.”

“We’re moving toward this period where we’re talking about the pandemic endgame, but we’re so far away from that. And I do worry,” Tuite said. “The CDC data ... really brings home the fact that we’re still in the middle of a pandemic.”

But experts stressed that what the CDC data does not do is suggest that vaccines have stopped working — and, in fact, the heightened risk of Delta only makes vaccination all the more urgent.

Vaccines are still extremely good at protecting people from severe hospitalization and death, which was always the intended goal of vaccines, Juni said.

In the CDC presentation, the public health agency estimates that fully vaccinated Americans have a 25-fold decreased risk of hospitalizations and deaths compared to those who are unvaccinated.

In Canada, only 0.7 per cent of hospitalized cases are in fully vaccinated patients, officials with the Public Health Agency of Canada said Friday. Breakthrough cases also remain exceedingly rare in Canada, comprising only 0.5 per cent of all cases between mid-December and July 12.

It’s true that 74 per cent of people infected in the Barnstable County outbreak were fully vaccinated, but this also doesn’t suggest that vaccines have stopped working, said Angela Rasmussen, a virologist with the Vaccine and Infectious Disease Organization at the University of Saskatchewan.

She pointed out the outbreak occurred during a holiday period where many tourists were streaming into the Barnstable County community, many of whom likely brought Delta along.

The July 2021 outbreak coincided with the Fourth of July; gay rights activist Peter Staley also pointed out on Twitter that 85 per cent of cases were male and the outbreak occurred during “Bear Week” in Provincetown, a festival popular with gay men.

It’s already known that vaccines are not 100 per cent effective at preventing infection, especially when it comes to Delta. So in a place like Barnstable County — which has one of the highest vaccination rates in the U.S. — it’s expected that an influx of Delta-infected people would trigger outbreaks, and that a proportion of cases would involve fully vaccinated residents.

What we don’t know is the total number of vaccinated people who were exposed to the outbreak but did not get infected, Rasmussen said. And if the same outbreak had occurred in a state with low vaccination rates, there would have been much higher rates of hospitalization and death, she added. In the Barnstable outbreak, only five people were hospitalized, with no deaths.

“A vaccine is kind of like hip waders,” Rasmussen said. “You can walk through a river, but if you eventually get deep enough, water is going to start pouring in over the top. And that’s kind of, I think, what happened in that community.”

Rasmussen said, however, that the CDC’s new findings are a “game-changer” in the sense that “we weren’t worrying a lot about transmission from vaccinated people, and now we do have to worry about that.” This

means the risk is now even higher for unvaccinated people, and there is even greater urgency to increase vaccination rates by reducing barriers and providing supports to people who may have trouble accessing doses.

For Juni, the new CDC data further underscores the critical importance of proceeding slowly and cautiously with Ontario’s next steps in reopening.

So far, the province has done an excellent job of controlling its Delta wave compared to other international jurisdictions, he said.

Next to Singapore, Ontario is the only place in the world that’s “kept Delta in check” after the variant became the dominant strain locally, according to Juni.

He said for now, Ontario is still in a “honeymoon” period where infection rates are low enough for fully vaccinated friends and family to continue gathering indoors, in small numbers, without their masks on.

But looking ahead to the fall and winter, people should mentally prepare themselves for the idea that public health measures, such as masks, will still be necessary for the foreseeable future — especially in September, when kids return to the classroom.

This is a difficult, but necessary, message, Juni said.

“I’m tired too. I would like this to be over with. Only the virus doesn’t care,” he said. “We’re in a really good spot and we don’t want to endanger that. If we keep doing the right thing — and not rush further and keep masking — that’s an investment for more liberty.”

“This data is the nail in the coffin for any suggestion that we can go back to normal and drop masks in the foreseeable future.” DR. PETER JUNI

ONTARIO COVID-19 SCIENCE ADVISORY TABLE