How much less likely are you to spread covid-19 if you're vaccinated?

A man is vaccinated in LA Jason Armond/ Los Angeles Times via Getty Images
Numerous studies have shown that people who are fully vaccinated against the covid-19 virus are less likely to infect others despite the introduction of the delta variant. These findings disprove the notion that vaccines don't do enough to stop the spread of coronavirus, which is a common belief in some circles.

They do indeed reduce transmission, according to Christopher Byron Brooke from the University of Illinois Urbana-Champaign. Although vaccine-vaccinated people can transmit the virus in certain cases, the data is so clear that the risk of transmission to vaccinated individuals is much lower than those who are not vaccinated.

Recent research has shown that people who have been vaccinated against the delta variant of the disease are 63% less likely to infect others than those who have not.

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Brechje De Gier, a researcher at the Center for Epidemiology and Surveillance of Infectious Diseases (Netherlands), said that this is only slightly less than the alpha variant. Her previous research had shown that people who were vaccinated against alpha are 73% less likely to infect others.

De Gier states that vaccines have a greater effect on transmission than 63%, and most people who are vaccinated don't get infected.

De Gier and her colleagues used data from the Netherlands contact trace system to calculate the secondary attack rate, the percentage of contacts infected with positive cases. After adjusting for age, they calculated how much vaccination could reduce this.

De Gier claims they can't calculate the total reduction in transmission from vaccination because they don't know how vaccination affects infection risk. Even if vaccination reduces the chance of infection by half, this would still mean that vaccines reduce transmission by over 80 percent overall.

Others have discovered the full effect. Ottavia Prunas, a Yale University researcher, applied two models to data from Israel earlier this year. This was where the Pfizer vaccine was first used. The overall vaccine effectiveness against transmission was 89% according to her teams.

The data was only available up to March 24, before delta took over. Prunas says that the team now uses more recent data to determine the impact of delta.

According to news reports from July, vaccines may not be as effective in preventing transmission. This is based on the claim that people who are vaccinated can contract as many viruses as those who aren't. Even if true, vaccines could still reduce transmission by preventing infections from occurring in the first place.

The news report that caused the sensation didn't actually measure the virus count in a person directly, but instead used Ct scores to measure viral RNA. This RNA can be derived from viruses that have been destroyed by the immune systems. Timothy Peto, University of Oxford, says that you can measure the RNA but it is rendered useless.

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Multiple evidence points out that Ct scores don't accurately reflect the virus level of an individual. First, infected people who have been vaccinated are less likely to spread the virus to others. Peto did a similar study as de Gier, using contact tracing data in England. He got similar results.

Petos' team also showed that infectiousness and Ct scores are not directly related. People who had been positive for vaccination showed the same viral load as those who weren't. They were thought to be just as infectious. Peto says that you're actually less infectious than we thought. Thats quite important. People were over-pessimistic.

Brooke's study provides yet another piece of evidence. Brooke's team collected samples from 23 people each day starting after the first positive test and continuing until the infection clears. They also performed tests including infecting cells in a dish.

Five out of six people who were fully vaccinated did not have any infectious diseases, unlike many people who are unvaccinated. Brooke says that the study showed that people who have been vaccinated shed less virus and stop shedding more quickly than people who are not vaccinated.

One thing is not good news: a Petos study has shown that the protection that a vaccine offers against infected people infecting others decreases over time. It decreased by about 25% over the three-month period following a second dose. He says that this has made him a strong believer in boosters. Given that we are currently in the middle of major epidemics [in the UK], they should get on with it.

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