Does being vaccinated against COVID-19 stop you getting infected?

People who are fully vaccinated against COVID-19 are far less likely to become infected and infect others, even with the arrival of the delta and delta plus variants. As cases rise sharply in the UK, however, fears are spreading over how effective vaccination is against the virus. But what does the research suggest - and what do experts say about the efficacy of the COVID-19 vaccines?

Does being fully vaccinated offer good protection against COVID-19 infection?

The COVID-19 vaccination booster programme is well underway in the UK, with an increasing number of eligible people being offered their third jab. According to NHS England, more than five million people have had a third vaccine since the vaccination programme began administering them last month.

Multiple studies suggest being vaccinated significantly reduces the chance of testing positive for COVID-19. In October, the results of an ONS COVID-19 infection study showed that being fully vaccinated provided the best protection. Higher protection still was found among those who had experienced a past infection followed by two vaccines.

Two doses of either the Pfizer or the AstraZeneca vaccine provided a similar level of protection to prior natural infection when the Delta variant was dominant, the research found. However, it also showed that vaccines were slightly less effective against the Delta variant compared with the Alpha variant.

Earlier this year, a separate study led by Imperial College London found that double vaccinated people were three times less likely than unvaccinated people to test positive for coronavirus.

"All COVID-19 vaccines being used in the UK protect people against getting infected with COVID-19, including against Delta," says Mark Jit, professor of vaccine epidemiology at the London School of Hygiene and Tropical Medicine. "This has been shown in both clinical trials and in studies looking at the impact that vaccines have had after they were widely introduced.

"The vaccines give much better protection against severe disease than against mild disease or infection with no symptoms, so some vaccinated people will still get infected," he adds. "But they get infected at a lower rate than unvaccinated people."

COVID-19 vaccination helps to prevent hospitalisation

Recently, Dutch researchers found that vaccinated people infected with the Delta variant are 63% less likely to infect people who are unvaccinated. This is only slightly lower than with the Alpha variant.

"The strength of this protection depends on the outcome you are looking at," says Brechje de Gier, who led the study at the National Institute for Public Health and the Environment in the Netherlands.

"The vaccines protect for around 60 to 80+% against infection with SARS-CoV-2 but for over 90% against hospitalisation with COVID-19. In my country, protection is 95% against hospitalisation and 97% against ICU admission," she explains.

"This means that while the protection against infection is a bit lower - for Delta compared to Alpha and possibly with more time since vaccination - these vaccine breakthrough infections tend to be milder and not require hospitalisation."

Why are vaccinated people still catching COVID-19?

COVID-19 vaccines are very effective, but none is 100% so. This is because immune responses vary from person to person and the virus that causes COVID-19 mutates and evolves, like the flu virus. Therefore, some people who are double vaccinated may still get infected - known as 'breakthrough infections'- but they are less likely to become very ill or die.

"The vaccines we have were developed against the original strain of COVID-19," explains Jit. "Variants such as Delta have mutations that make it harder for the immune systems of vaccinated people to recognise them. But they are still similar enough that the vaccines give good protection against them, especially against severe disease."

Cases of COVID-19 are also rising among the vaccinated because the number of people in the UK who have had both doses is continuing to rise. As more of the population is vaccinated, the relative proportion of those with COVID-19 who have had both jabs will also increase.

Additionally, studies have shown that immunity among vaccinated people begins to wane over time. Although those who are vaccinated are significantly less likely to develop severe COVID-19 than unvaccinated people, their immunity can still decrease over time. This is why booster jabs are necessary, especially for vulnerable or high-risk people.

Vaccines don't need to be 100% effective to work

Professor Ian Jones, a professor of virology at the University of Reading, explains that a vaccine's ability to block transmission doesn't need to be 100% to make a significant difference.

"Vaccination will lessen the chance of catching COVID-19 but not prevent it entirely," he explains. "Its real purpose is to prevent serious disease rather than formal infection and it does this very well. Many vaccines work in a similar way, and COVID vaccines are not unusual in that respect.

"The Delta variant is an evolved version of the virus which is better at getting into human cells whereas the vaccines were developed for the original virus," Jones adds. "This slight mismatch means that infection is easier for the Delta virus even when immunity is present. However, protection against severe disease remains just as high."

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