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Pfizer’s vaccine appears to reduce coronavirus transmission

Researchers are seeing the first real-world suggestions that a vaccine will curb the spread of the coronavirus, not just save individuals from getting severely sick.

In two separate preliminary findings released on February 8 at, researchers from Israel report that people vaccinated with Pfizer's vaccines and who still get infected with the coronavirus carry less virus in their bodies than unvaccinated infected people.

If the vaccines limit the spread of the virus,' it means that even people who are not vaccinated will receive immunity from the people around them who are vaccinated,' says Marm Kilpatrick, a specialist on infectious diseases at the University of California, Santa Cruz. They're not going to kill as many more people as they could have before the injection, he adds, as more people get vaccinated.

And as details on the potential of vaccination to curb infection are just beginning to surface, U.S. public health authorities have revised quarantine protocols for people exposed to the virus who are vaccinated. If the infection persists after receiving all doses of Pfizer's or Moderna's vaccine for two weeks to three months, no quarantine is required, the U.S. February 10, the Centers for Disease Control and Prevention said. That's because the vaccines are highly effective at reducing the effects of COVID-19 and it is suspected that those who get ill are more likely to spread the virus.

The recent discovery that coronavirus transmission can be curbed by vaccines comes from Israel's vaccination and infection info. After a clinical trial found that Pfizer's vaccination was 95 percent effective in preventing symptoms of COVID-19, a significant section of the country's population was quickly vaccinated by Israeli health officials, particularly people 60 years and older (SN: 11/18/20). According to data from Our World of Data, as of February 6, 80% of people in the age group have received both doses of the vaccine, compared to 20% of younger people. At least one dosage has been taken by almost half of the population, and the incidence of infection and hospitalization in Israel is dropping.

Researchers compared the levels of the virus in more than 15,000 coronavirus research samples, eager to see whether the vaccine campaign was to curb the viral spread. The vaccine history of those examined was not identified by the team. But Ella Petter, a machine biologist with MyHeritage Lab, a genealogy firm based in Or Yehuda, Israel, who performs COVID-19 tests, says that 'there was a peculiar moment in time when we had two separate groups' in the population, with older people possibly having been vaccinated and younger people most likely not.

Petter and her colleagues found that samples who tested positive for coronavirus from people over the age of 60 had lower coronavirus levels on average than samples from people 40 to 60 years of age who were less likely to have been vaccinated. Computational studies found that the discrepancies between the two classes were better clarified by the vaccine rollout, rather than variables such as populations or novel strains of coronavirus (SN: 2/5/21).

Researchers observed in a different study that individuals who were infected within 12 days after their first Pfizer shot and had little or no defense against COVID-19 harbor four times more coronavirus in their bodies compared to individuals who were infected more than 12 days after their first shot.

Past experiments have shown that a person is less likely to pass on coronavirus to others compared with individuals with a higher viral load when there is less virus in the body. Taken with the recent results that indicate that vaccinated patients who are still infected have the fewer virus than unvaccinated people, the knowledge indicates that the coronavirus transmission may be decreased by Pfizer's vaccine.

While vaccinated individuals tend to be less likely to spread the virus, how much transmission might still occur remains uncertain. It is because the viral load of individuals can differ greatly in general; the drop in viral loads reported in vaccinated individuals is slight relative to the range of viral levels experts have seen in patients with COVID-19, Kilpatrick says.

It is also important to remember that the studies display "a viral load snapshot," which indicates how much virus a person has borne, he says, at a single point in time. Analyzing the viral load over an entire infection would demonstrate how infectiousness could fluctuate over that time, from the first positive test to recovery. This could help researchers to understand the consequences of vaccines, such as how long it could be infectious for a person who contracted the virus before being completely vaccinated by a vaccine.

And it's uncertain if the second dose, which provides an additional boost to the immune system, will decrease vaccinated people's viral loads more, Kilpatrick says.

Even, the lower viral loads found in the coronavirus test samples mean that even if vaccinated individuals become infected, respiratory droplets and aerosols can release less virus into the atmosphere. And that means that vaccinated patients could be less infectious since the research only looked at individuals who obtained Pfizer's vaccine. This could hopefully speed up the end of the pandemic and a return to a form of normalcy, if real.

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