How do Covid vaccines change your risk of infection? And are you less likely to pass it on?

Explainer: Reports of vaccinated people getting infected with Covid-19 might have you wondering how well the vaccines protect you against coronavirus, and how long that protection lasts.

Health authorities in the UK have begun offering a third dose of the Covid-19 jab to people over the age of 50, and those at an increased risk of being infected or getting seriously ill, in a bid to improve their protection.

Similarly, in the US, an FDA advisory panel has recommended booster vaccines be given to all Americans over the age of 65 and those at high risk of severe disease.

So, how well are the Covid-19 vaccines working at preventing people from getting sick?

And if you do get infected after being vaccinated, how likely are you to pass the virus onto someone else?

Vaccines highly effective at preventing death

Research shows Covid-19 vaccines remain highly effective at preventing severe disease and death six months after vaccination, including against the highly transmissible Delta variant.

"Consistently, all of the vaccines seem to be doing that with more than 90 per cent effectiveness, including with time," said Peter Collignon, an infectious diseases physician and microbiologist at the Australian National University.

"[They're] even better at reducing your risk of dying - probably a 95 to 98 per cent decreased risk."

A recent study of more than 44,000 people in Los Angeles found unvaccinated people were 29 times more likely than vaccinated people to end up hospitalised from Covid-19.

The rate of hospitalisation among vaccinated people was 1 per 100,000 people.

In Australia, we're seeing a similar pattern: the vast majority of hospitalisations and deaths from Covid-19 are occurring in unvaccinated people.

"This is currently an epidemic of the unvaccinated," Professor Collignon said.

"They're the main people getting infected, and spreading it. That's why vaccination is so important."

What about breakthrough infections?

While vaccination is our strongest weapon against Covid-19, no vaccine is 100 per cent effective, and some fully vaccinated people will become infected.

These are known as breakthrough infections.

Estimates vary, but UK data suggests 0.2 per cent of the population - or one person in every 500 - experiences a breakthrough infection once being fully vaccinated.

Fortunately, most people who get Covid-19 after being vaccinated won't become very sick, and even fewer will require hospitalisation, said epidemiologist and biostatistician Adrian Esterman from the University of South Australia.

"The vast majority of people will only have very mild symptoms, if any at all," he said.

The people most at risk of serious illness from a breakthrough infection are adults over 65 and people with compromised immune systems.

That's because they tend to generate a weaker immune response to vaccines, and is why they're being prioritised for booster vaccines overseas.

Data from the Centers for Disease Control and Prevention (CDC) shows that between April and June this year, fully vaccinated people accounted for 7 per cent of all Covid-19 hospitalisations in the US.

As the Delta variant surged in June and July, this figure jumped to 14 per cent - but experts say this increase is to be expected.

As vaccination rates grow, the proportion of fully vaccinated people who get infected - and the very small proportion who get seriously ill - will also increase, especially with the surging spread of Delta.

In Israel, rates of hospitalisations and ICU admissions have increased among vaccinated people (predominantly among the elderly), but their risk of being hospitalised is still reduced 40-fold compared to those who are unvaccinated.

How likely am I to get infected?

According to the UK's Office for National Statistics, the AstraZeneca vaccine is 67 per cent effective against infection with Delta, while the Pfizer jab is 80 per cent effective.

Other preliminary research suggests there isn't much difference between the two, and that they both reduce the risk of infection by 80 per cent.

Estimates for vaccine effectiveness against infection vary considerably from country to country, and depend on the type of vaccine, the prevailing Covid-19 variant, and the time elapsed since the second vaccine.

Estimates vary from around 55 to 80 per cent, with most hovering around the 60-70 per cent mark.

While experts agree that vaccine-induced immunity against infection is likely to wane over time, breakthrough infections may also be the result of looser Covid-19 restrictions and increased socialising.

We also know early vaccine recipients tended to be older, have underlying health conditions, or work in high-risk professions - putting them at higher risk of getting infected.

What's clear is that vaccinated people still have a considerably lower chance of getting infected than unvaccinated people, which in turn, cuts their risk of passing the virus on.

Vaccinated people less likely to spread virus

Real-world evidence shows vaccinated people are able to transmit Covid-19 to others, but it's thought their risk of doing so is substantially reduced.

"For starters, [vaccinated people] have decreased their risk of giving Covid-19 to others because they've reduced their risk of getting infected in the first place," Professor Collignon said.

"Secondly, [if they do get infected], they tend to have milder disease and have it for a shorter period of time, which also decreases their risk."

In July, a study published by the CDC found vaccinated people infected with the Delta variant carried roughly the same viral load in their noses and throats as unvaccinated people, sparking concern they could spread the virus just as easily.

But subsequent research has found this genetic material declined faster in vaccinated people, meaning they likely spread the virus for less time, Professor Esterman said.

"There's also more recent data now that shows if you're fully vaccinated, you're likely to have a lower viral load," he said.

According to the CDC, it's still not clear whether fully vaccinated people with asymptomatic infections can transmit the virus.

What is clear, however, is that unvaccinated people are still the major drivers of transmission.

In the US, infection rates in the least vaccinated states are roughly four times as high as in the most vaccinated states.

Professor Esterman said the benefits of vaccination went well beyond the individual.

"If you're fully vaccinated, you're less likely to pass Covid-19 onto others, including your own family and the general population."

- ABC