A vaccine breakthrough case is defined as the detection of SARS-CoV-2 infection in individuals, who have been fully vaccinated against COVID-19. But what does this mean, what are the risks and what should a person do if they contract COVID-19 between vaccine doses?

The vaccines are not 100% effective, so we would expect breakthrough infections. The majority of these will be mild. For the Sisonke study, so far 94% of breakthroughs were mild, 4% moderate and 2% severe.

When the rate of infection is very high there will be more breakthrough infections. Once population immunity levels rise with increased vaccination, there will be less transmission of the virus and fewer breakthrough infections. Any events that happen after vaccination must be reported so that these can be investigated. There is a Medsafety app and details can be found by visiting

The vaccine will not cause a positive COVID-19 test. They were most likely in the incubation period of COVID-19 infection when they were vaccinated. This is especially true when infection rates are high. 

No, COVID-19 infections within 2 weeks of vaccination are not considered breakthrough infections. It takes at least 2 weeks for the body to generate an immune response to a vaccine.

In the Sisonke study, they are considering a breakthrough infection as >28 days post-vaccination. The CDC defines a breakthrough case as an infection >14 days after FULL vaccination.

If someone tests positive for COVID-19 after vaccination (breakthrough infection), they should isolate as usual. The amount of viruses carried by people with breakthrough infections is low but it may still be possible to transmit the virus.

At this stage, there is no evidence to suggest that vaccinated individuals have different types of symptoms if they become infected compared to unvaccinated. Symptoms will, however, be milder in vaccinated individuals compared to those who are unvaccinated and have a shorter duration. Asymptomatic infections among vaccinated people will also occur.

Breakthrough infections are currently being evaluated for clustering by patient demographics and co-morbidities, geographic location, time since vaccination, vaccine type, and SARS-CoV-2 lineage. Risk factors for severe disease are likely to be similar in vaccinated and unvaccinated individuals.

No, there should not be any conflict with existing natural antibodies. There is still much to learn about these immune responses. A natural immune response might be variable, so there is a benefit in boosting the natural immunity with the vaccine.

The Johnson and Johnson vaccine has shown good protection against severe COVID-19 disease after a single dose. At this stage, more people need to get their primary vaccine dose, before booster doses are started.

Only 2% of the breakthrough infections in the Sisonke trial were severe. Doctors should still use PPE, even if they have been vaccinated.