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Can Covid-19 vaccinated people still spread the coronavirus?

The spread of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the causative agent of Covid-19, has resulted in an unprecedented global public health and economic crisis. While the development of vaccines has gone a long way to reduce the severity of the disease, reducing hospital admissions, use of critical care beds and death as a direct result of the Covid-19 pandemic, there are major questions still to be answered, one being: Can Covid-19 vaccinated people still spread the coronavirus?

Studies to date demonstrate an effectiveness of the authorised Covid-19 vaccines in reducing severe illness, hospital admissions and use of ICU beds. A large scale South African study (Sisonke) found the Johnson & Johnson’s Covid-19 vaccine to be 71 percent effective at preventing hospitalisations and 96 percent effective at preventing death from the Delta variant.


Vaccination at the Steve Biko Academic Hospital in Pretoria | Phill Magakoe/AFP via Getty Images

The Sisonke study involved over 477,000 healthcare workers who received the single-shot Covid-19 vaccine between February and May 2021. Workers were exposed to both the Beta and Delta variants of Covid-19 as they spread throughout the country.

The Johnson & Johnson shot provided protection against both variants though there was slightly “greater efficacy against Delta than against Beta”; the study indicated that vaccinated population groups were better protected compared to unvaccinated populations according to Glenda Gray, the president of the South African Medical Research Council and co-leader of the trial.

Now, knowing if Covid-19 vaccinated people can still spread the coronavirus will be important in informing government decisions on public health. We are aware that some people cannot be vaccinated because of underlying conditions and others with weakened immune systems, from cancer or medical treatments, may not be fully protected by their vaccinations, and vaccines are yet to be authorised for children under the age of 12 years. 

Understanding vaccine efficacy and effectiveness gives us an idea when it comes to choosing which vaccine to take, if at all we choose to have one. Vaccine efficacy refers to how well a vaccine performs in a carefully controlled clinical trial, whereas effectiveness describes its performance in real-world observational studies.

Studies in Israel, the United States and the United Kingdom have given a body of evidence demonstrating that the authorised Covid-19 vaccines are both efficacious and effective against symptomatic, laboratory-confirmed Covid-19, including severe forms of the disease. Furthermore, a growing body of evidence suggests that mRNA Covid-19 vaccines also reduce asymptomatic infection and transmission. Substantial reductions in SARS-CoV-2 infections (both symptomatic and asymptomatic) will reduce overall levels of disease, and therefore, viral transmission.

In one study, the US Centers for Disease Control and Prevention (CDC) tested volunteer health care personnel and other front-line workers at eight United States locations for SARS-CoV-2 infections weekly for three months, regardless of symptoms or vaccination status. Researchers found that fully immunised participants were 25 times less likely to test positive for Covid-19 than were those who were unvaccinated.

General understanding so far about the authorised Covid-19 vaccines is that breakthrough infections (infection of vaccinated persons) have been detected. A study of vaccinated people led by King’s College London found some people were more at risk of breakthrough infections than others, these included older adults and people living in deprived areas, and this was particularly evident in people who had only had one jab.

Outbreak of infections have been reported among some highly vaccinated and highly tested population groups including professional sports teams and frontline workers. Reported infections among the vaccinated New York Yankees and an identical outbreak in airport workers in Singapore show that even among the fully vaccinated, new and more infectious variants can spread fast.

Indeed latest data from Covid-19 tests in the United States, the United Kingdom and Singapore show that vaccinated people who become infected with Delta SARS-CoV-2 can carry as much virus in their nose as do unvaccinated people. This means that despite the protection offered by vaccines, a proportion of vaccinated people can still pass on Delta variant.

We can say with a high degree of certainty that vaccinated and asymptomatic people not only can still get infected, they might also transmit the coronavirus to close contacts. Disappointing as it is, this is normal and expected, after all vaccines are only 95% efficacious and that too against symptomatic disease. The full efficacy of the Covid-19 vaccines occurs after a couple of weeks post second vaccination.

There are however, some encouraging signs. A yet to be published and peer-reviewed study suggests that the Moderna mRNA Covid-19 vaccine can produce coronavirus-fighting antibodies in the oral and nasal fluid. Arguably, since that is where SARS-CoV-2 makes its entry, antibodies in the mouth and nose should block the virus from getting into the body, effectively providing “sterilising immunity.” This would also mean vaccinated people probably would not spread the virus through respiratory droplets.

As with other vaccines before it, the hope had been for the Covid-19 vaccines to prevent illness and achieve “sterilising immunity,” where the vaccination blocks the germ from even being able to get into the body at all. This sterilising immunity means someone who is vaccinated will neither catch the virus nor transmit it further. However, it must be understood that the primary objective of a vaccine is to prevent illness thus, for a vaccine to be effective it does not need to prevent the germ from infecting an immunised person.


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