Evidence mounts that people with breakthrough infections can spread Delta easily

A preliminary study has shown that in the case of a breakthrough infection, the Delta variant is able to grow in the noses of vaccinated people to the same degree as if they were not vaccinated at all. The virus that grows is just as infectious as that in unvaccinated people, meaning vaccinated people can transmit the virus and infect others.

Previous studies in hospitals in India; Provincetown, Massachusetts; and Finland have also shown that after vaccine breakthrough infections with Delta, there can be high levels of virus in people’s nose whether they are vaccinated or not. The next logical step was to determine whether vaccinated people could shed infectious virus. Many experts suspected they did, but until this study it hadn’t been proven in the lab.

“We're the first to demonstrate, as far as I'm aware, that infectious virus can be cultured from the fully vaccinated infections,” says Kasen Riemersma, a virologist at University of Wisconsin who is one of the authors of the study.

“Delta is breaking through more preferentially after vaccines as compared to the non-Delta variants” because it’s extremely infectious and evades the immune response, says Ravindra Gupta, a microbiologist at University of Cambridge. Gupta’s lab was one of the first to document that fully vaccinated healthcare workers could get infected with Delta and had high levels of virus in their noses.

If the Wisconsin study finding holds up, then people with breakthrough infections—many of whom do not develop COVID symptoms—can unknowingly spread the virus. “It [is] an alarming finding,” explains Katarina Grande, a public health supervisor and the COVID-19 Data Team Lead of Madison & Dane County, who led the study.

What concerns Eric Topol, the founder and director of the Scripps Research Translational Institute, is that fully vaccinated individuals who are infected with the Delta variant can transmit the virus and this can happen at a higher rate than previous strains in the days before symptoms, or in the absence of symptoms. “Which is why masks and mitigation measures are important, even for people [who are] vaccinated,” he says.

Studies like these highlight that transmission of the Delta variant can be much higher that currently estimated, according to Ethan Berke, chief public health officer of the UnitedHealth Group. Berke’s research has shown that frequent testing with rapid results, even if preliminary, can be very effective in curtailing the COVID-19 pandemic. Berke was not involved in the Wisconsin study.

“Even though the study was based on one region, it offers important insight into how people can spread the virus to others whether they’re fully vaccinated or not. This sort of insight, especially as it’s tested and refined, is incredibly helpful as organizations develop policies around testing, social distancing, and vaccinations,” Berke says.

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How do we know the virus in the sample is infectious?

To test for SARS-CoV-2, the scientists employed a measurement called threshold cycle (Ct) that uses glowing dyes to reveal the quantity of viral RNA in the nose.

“SARS-CoV-2 virus infects nose and upper airway. It is very difficult to get a very high level of antibodies for long periods of time in that area. The immune system is not really designed to put high levels of antibodies at those sites,” Gupta says.

Ct values correlate with the viral load, which is the number of viral particles present in the body. When the quantity of virus passes a certain threshold, researchers expect an infected person to shed SARS-CoV-2 and potentially infect others. The Wisconsin study analyzed the nasal swabs from 719 cases of unvaccinated and fully vaccinated people who had all tested positive and found that 68 percent of the studied breakthrough patients had very high viral loads. High viral load is a sign that the virus is replicating, Gupta says.

To discover whether the nasal swabs had infectious virus, the Wisconsin researchers grew virus from 55 patient samples (from both vaccinated and unvaccinated people who tested positive) in special cells prone to SARS-CoV-2 infection. Grande’s team detected infectious virus in nearly everyone: from 88 percent of unvaccinated individuals and 95 percent of vaccinated people.

“We put the samples onto cells, and the cells died when they got infected. And so that clearly demonstrates that there is virus there, and that it's infectious,” Riemersma explains.

If vaccinated people can still produce a lot of infectious viruses, it means they can spread the virus as easily as those who are not vaccinated.

Masks and vaccination needed to prevent viral transmission 

“We have kind of a perfect storm of multiple things going on: super-infectious variants, really susceptible population, debates around masking,” Grande says.

More than 93.8 percent of the U.S. is at substantial or high level of risk for community transmission of SARS-CoV-2, according to the Centers for Disease Control and Prevention (CDC). CDC defines an area to be at high risk when either the number of new cases in a county exceeds 100 per 100,000 persons, or more than 10 percent of COVID-19 tests come back positive in the past seven days. In those areas, CDC recommends wearing a mask indoors in public to maximize protection from the Delta variant and prevent spreading it to others.

Although authorized vaccines prevent severe COVID-19 and death, they offer substantially lower protection among older people, those with weakened immune system, or with an underlying medical condition.

“We need more information about the Delta variant to better understand how it works, can be transmitted, and ultimately informs how we protect ourselves at home, work and within our communities,” Berke says. “In the meantime, basic hygiene, including masking, social distancing, regular testing and vaccines will continue playing a vital role in slowing transmission and preventing serious illness and death.”