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The Pfizer vaccine appears to reduce coronavirus transmission

Researchers are receiving the first real-world indications that a vaccine can slow the spread of coronavirus and not just prevent people from becoming seriously ill.

People vaccinated with Pfizer shots and still infected with the coronavirus carry fewer viruses in their bodies than unvaccinated people who are infected, Israeli researchers report in two separate preliminary studies published Feb. 8 on

If vaccines reduce the spread of the virus, "it means that even people (who are not vaccinated) will get protection from vaccinated people around them," says Marm Kilpatrick, an infectious disease researcher at the University of California, Santa Cruz. As more people are vaccinated, they will not continue to infect as many other people as before the shot, he says.

Even as data on the ability of vaccines to slow transmission is beginning to emerge, U.S. public health officials have updated quarantine guidelines for vaccinated people exposed to the virus. If exposure occurs two weeks to three months after receiving both Pfizer or Moderna vaccines, no quarantine is needed, the U.S. Centers for Disease Control and Prevention said Feb. 10 because the vaccines are very effective for prevent the symptoms of COVID-19, and it is believed that people who get sick are more likely to transmit the virus.

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The new discovery that vaccines can slow the spread of coronavirus comes from Israel’s vaccination and infection data. After a clinical trial showed that the Pfizer vaccine was 95 percent effective in preventing COVID-19 symptoms, Israeli health officials quickly vaccinated a large portion of the country’s population, especially people 60 and older (SN: 11/18/20). As of Feb. 6, 80 percent of people in that age group had received both doses of the vaccine compared to 20 percent of younger people, according to data from Our World in Data. Nearly half of the population has received at least one dose and rates of infections and hospitalizations in Israel are falling.

Curious to see if the vaccination effort was slowing down viral transmission, the researchers compared virus levels in more than 15,000 coronavirus test samples. The team was unaware of the vaccination status of the people tested. But "this was a unique time when we had two different groups" in the population, probably the older ones were vaccinated and the younger ones probably not, says Ella Petter, a computer biologist at MyHeritage Lab, a genealogy company based in O Yehuda. Israel, which conducts COVID-19 tests.

Petter and colleagues found that samples that tested positive for coronavirus in people over 60 had lower levels of coronavirus on average than samples from people in their 40s and 60s, who were less likely to be vaccinated. Computational analyzes showed that vaccine release better explained the differences between the two groups, rather than factors such as demographics or new coronavirus variants (SN: 2/5/21).

In a separate study, researchers found that people who became infected within twelve days of their first shot with Pfizer – and had little or no protection against COVID-19 – harbor four times more coronavirus in their bodies compared to people who they became infected more than 12 days after their first shot.

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Past studies have shown that when there are fewer viruses in the body, a person is less likely to transmit the coronavirus to other people compared to people with higher viral load. Taken with new findings that show that vaccinated people who are still infected have fewer viruses than unvaccinated people, the data indicate that the Pfizer vaccine can reduce the spread of coronavirus.

Although vaccinated people appear to be less likely to transmit the virus, it is still unclear how much transmission could occur. This is because people’s viral load can vary greatly in general; Kilpatrick says the decrease in viral loads in vaccinated people is small compared to the range of viral levels that experts have seen in patients with COVID-19.

It’s also important to note that studies show “a snapshot of viral loads,” which represents the amount of virus a person carried at a single time, he says. Analyzing the viral load of an entire infection, from the first positive test to recovery, would show how infectiousness can fluctuate during that time. This could help researchers better understand the effects of vaccination, such as how long it can take for a person to catch the virus before they are fully protected with a vaccine.

And it’s unclear whether the second dose, which gives the immune response an extra boost, could further reduce viral loads in vaccinated people, Kilpatrick says. It is also unknown how long the protection can last.

Still, those low viral loads observed in coronavirus testing samples suggest that even if vaccinated people become infected, they can release fewer viruses into the environment in respiratory droplets and aerosols. And that suggests that vaccinated people may be less contagious, although studies have only looked at people who have received the Pfizer vaccine. If true, this could help speed up the end of the pandemic and return to some kind of normalcy (SN: 29/1/21).

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