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What’s wrong with the claim that vaccinated people can spread COVID-19 just as readily as the unvaccinated

People looking to cast doubt on the vaccines’ efficacy often cite a CDC study leaked by the New York Times saying viral loads were similar for vaccinated and unvaccinated people. But the study has serious limitations, and in any case doesn’t give the full picture on transmission.

covid vaccination
A person receives a shot of coronavirus disease vaccine at a community vaccination event in Martinsburg, West Virginia.
REUTERS/Kevin Lamarque

Early this summer, as the number of Minnesotans reaching full vaccination status increased rapidly and COVID-19 case numbers dropped, things seemed to be looking better in the pandemic forecast. Many Minnesotans who hadn’t traveled or gone to the grocery store maskless in more than a year started loosening up on pandemic restrictions. Things felt almost normal.

Then came the delta variant and reports of a COVID-19 outbreak among mostly vaccinated people in Provincetown, Mass. in July. Leaked Centers for Disease Control slides suggested the delta variant may be similarly transmissible in vaccinated and unvaccinated people.

Ever since, people skeptical of the vaccines or their efficacy, and some chafing against vaccine mandates have cited the CDC’s report and others with similar findings.

“It’s my belief that vaccinated individuals are spreading COVID and catching COVID,” one state employee said in a Minnesota Senate committee last week, arguing for frequent testing for both vaccinated and unvaccinated state employees covered by the state’s vaccine requirement.

Following her testimony, Minnesota Sen. Jim Abeler held up an article citing CDC research. “It says in there, fully vaccinated people with delta variant breakthrough infections can spread the virus to others,” he said, also citing research in Israel that found breakthrough infectious among the vaccinated.

State Sen. Jim Abeler
State Sen. Jim Abeler
It’s true the CDC study found similar levels of virus in vaccinated and unvaccinated people that had contracted COVID-19. But that study isn’t the final word on transmission of COVID-19: first, it had limitations in the way it was conducted that could have missed important distinctions between vaccinated and unvaccinated COVID-19 carriers. But even if it has been perfect, focusing on breakthrough infections misses the bigger picture: that people who are vaccinated are far less likely to get infected in the first place.

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Limitations of the CDC study

Dr. Jill Foster, a pediatric infectious disease physician at the University of Minnesota Medical School and M Health Fairview, noted some limitations of the CDC study.

The study measured the viral load in specimens from vaccinated and unvaccinated people with COVID-19 infections. Levels of virus were measured using a metric called cycle threshold, which refers to the number of times a test machine has to run to find viral particles. The fewer the cycles, the more virus that’s in the sample. The cycle threshold — or number of times the machine had to run — to find viral particles was similar in vaccinated and unvaccinated specimens.

Dr. Jill Foster
Dr. Jill Foster
But there are some issues with making assumptions about COVID-19’s behavior in vaccinated and unvaccinated people based on this measure.

“When I swab your nose and I swab somebody else’s nose, I might not get the same amount of material,” Foster said. If someone has a runny nose, a sample taken via nasal swab could be diluted, for example.

Furthermore, Foster said molecular tests that are used to detect COVID-19 are not good at detecting whether the viral particles in a sample are dead or alive like the cultured tests used for infections like strep throat, which grow out bacteria over time, are.

“[They] can’t differentiate between somebody who has live virus and their nose versus somebody who, the virus went into their nose and their immune system killed it,” Foster said. That factor is among the reasons many researchers are dubious that similar levels of virus found in vaccinated and unvaccinated samples indicate similar transmission patterns.

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A study released late last month that looked at COVID-19 transmission rates among people vaccinated with the Pfizer and AstraZeneca vaccines again acknowledged similar viral loads in samples taken from unvaccinated and vaccinated people with delta variant COVID-19 infections, but it also found transmission rates lower in the latter population.

Less likely to be infected

At some level, vaccinated and unvaccinated people can still spread COVID-19. However, leaving it at that is a massive oversimplification of the issue for another big reason: People who are vaccinated against COVID-19 are far less likely to be infected with the virus in the first place.

Of the more than 3.2 million Minnesotans who have been fully vaccinated against COVID-19 as of September 12, the most recent data available, 1.2 percent had confirmed breakthrough infections. Roughly six in 10,000 vaccinated people had ended up in the hospital with COVID-19, and seven in 100,000 people who have been fully vaccinated had died.

Unvaccinated, the risk of dying of COVID-19 is 11 times higher, the CDC found.

Foster said she likes to use a grocery store analogy to explain the relative risk of transmission by vaccinated versus unvaccinated people.

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If you live in an area where vaccination rates are high, on any given trip to the grocery store, many, many fewer of those patrons are likely to be infected with COVID-19. If they are infectious, their infectiousness period is likely to be shorter than an unvaccinated person. So you’re much safer at the grocery store in this scenario than in an area with more unvaccinated people, who are more likely to be infectious for longer.

A study released in August found vaccinated people clear the virus from their system more quickly than unvaccinated people, at an average of 5.5 days, compared to 7.5 days for the unvaccinated.

Vaccinated people should still be careful because they can still become infected with COVID-19 and transmit the virus to others. But breakthrough infections remain relatively rare in the first place, and they appear to be less likely to cause infections of others.

The bottom line, said Dr. Frank Rhame, Allina Health infectious disease specialist, is that vaccinations work.

“We clearly know that people who are vaccinated don’t get very sick and don’t die,” he said.