Vaccines’ primary purpose is to prevent people from getting really sick with the virus, and it quickly became clear the vaccines are highly efficient at doing just that. Efficacy against symptoms of the disease in clinical trials has ranged from 50% (Sinovac) to 95% (Pfizer/BioNTech), and similar effectiveness has been reported in the real world.
However, even the best vaccines we have are not perfect, which means some vaccinated people still end up catching the virus. We call these cases ‘breakthrough’ infections. Indeed, between 10 April and 1 May, six people in hotel quarantine in New South Wales tested positive for COVID-19, despite being fully vaccinated.
But how likely are vaccinated people to actually pass the virus on, if they do get infected? Evidence is increasing that, not only do COVID-19 vaccines either stop you getting sick or substantially reduce the severity of your symptoms, they are also likely to substantially reduce the chance of transmitting the virus to others.
But how does this work, and what does it mean for the pandemic?
Vaccinated people are much less likely to pass on the virus
Early evidence from testing in animals, where researchers can directly study transmission, suggested immunisation with COVID-19 vaccines could prevent animals passing on the virus.
But animals are not people, and the scientific community has been waiting for more conclusive studies in humans.
In April, Public Health England reported the results of a large study of COVID-19 transmission involving more than 365,000 households with a mix of vaccinated and unvaccinated members.
It found immunisation with either the Pfizer or AstraZeneca vaccine reduced the chance of onward virus transmission by 40–60%. This means that if someone became infected after being vaccinated, they were only around half as likely to pass their infection on to others compared to infected people who were not vaccinated.
