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Do COVID vaccines prevent transmission of the virus?

Tai Hai Chen

Oct 15, 2017

Studies show that while both the vaccinated and unvaccinated can have similar levels of the virus in their bodies, the vaccinated are less likely to pass it on to others. Plus, ‘COVID toes’ and a pill to treat COVID at home.

It has long been established that COVID-19 vaccines reduce the risk of serious illness and hospitalisation. But there has until recently been a question mark over whether they reduce transmission of the virus.

It is an important question and one around which public health policies have been built. The United Kingdom, for example, has mandated COVID vaccines for all social care staff to protect the vulnerable people they care for; they are set to do the same for NHS staff. And in Italy, from October 15, workers will have to show proof of vaccination, a negative COVID test or recovery from a recent infection to their employer. Anyone unable to do this risks being suspended from work without pay. Other countries are adopting similar measures.

But do vaccines actually limit the spread of the virus?

A large study, not yet peer-reviewed, led by a team at Oxford University and looking specifically at the Delta variant has shown that both the Pfizer and AstraZeneca vaccines do indeed reduce transmission of the disease. The study looked at almost 150,000 contacts who were traced from nearly 100,000 initial cases of COVID. The initial COVID-positive cases contained a mix of vaccinated and unvaccinated people and the aim was not only to see which groups were most likely to pass on the virus, but also which of the Pfizer or AstraZeneca vaccines were most effective in reducing transmission.

The findings showed that both vaccines reduced transmission, but that the Pfizer vaccine was the most effective in doing so. The contacts of those who were fully vaccinated with the Pfizer vaccine were 65 percent less likely to test positive for COVID-19 compared with the contacts of those who were unvaccinated. The contacts of those fully vaccinated with the AstraZeneca vaccine, meanwhile, were 36 percent less likely to test positive when compared with the contacts of those who were unvaccinated.

As with previous studies, this Oxford study found that the vaccinated and unvaccinated groups had similar levels of the virus in their bodies, but those who were vaccinated were less likely to pass it on to others, suggesting that they clear the virus quicker and are more likely to have less infectious viral particles.

It is important to remember that those who are vaccinated will have primed immune systems that will recognise the coronavirus far quicker and be able to rid their body of it faster than those who are unvaccinated and whose immune system will take time to respond to the virus.

All of this is good news, but there is a fly in the ointment. The study also found that the protection the vaccines offer against transmission wanes over time.

Three months after having the AstraZeneca vaccine, those who had breakthrough infections were just as likely to spread the Delta variant as the unvaccinated. While protection against transmission decreased in people who had received the Pfizer vaccine, there was still a benefit when compared with unvaccinated people. Although this appears disheartening, the vaccines still offer good protection against serious illness.

With booster vaccines well under way in many developed nations, it is likely they too will help reduce transmission, and whether or not their protection against transmission wanes over time remains to be seen.

With an increasing wealth of data to suggest vaccinations reduce the risk of transmitting COVID-19 to those around us, we may well see more countries adopt stricter measures to encourage the unvaccinated to take up the vaccines – not just for themselves but for the wider population. The truth of the matter is that it is going to be a combination of vaccines and public health measures that will see us out of this pandemic.

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