One Nation leader Pauline Hanson has claimed that vaccinated and unvaccinated people are equally likely to transmit the virus that causes COVID-19, meaning unvaccinated healthcare workers pose no additional workplace risk when compared to vaccinated colleagues.
The claim is false on two counts. Firstly, vaccinated people are less likely to be infected with SARS-CoV-2 than the unvaccinated, which means they are also less likely to transmit the virus. Secondly, multiple studies show that even when they do become infected, vaccinated people are less likely than those unvaccinated to pass on the virus to others.
Speaking in a Senate debate on November 22, Senator Hanson said: “A recent Lancet (medical journal) study shows that vaccination alone is not sufficient to prevent the transmission of the Delta variant. This means that both vaccinated and unvaccinated (people) are as contagious as each other. There’s no evidence proving that unvaccinated healthcare workers pose an additional risk” (page 3).
When asked which study Senator Hanson was referring to in her speech, the senator’s office told AAP FactCheck in an email that she was directly quoting a doctor with expertise in immunology, rather than directly highlighting the study itself, and therefore could not supply details of the relevant research.
However Chris Baker, a biosecurity risk statistician at the University of Melbourne, told AAP FactCheck that “whether healthcare workers or not”, unvaccinated people have a higher risk of transmitting the virus than vaccinated people.
There were two key aspects to determining the relative likelihood of vaccinated and unvaccinated people being contagious, Dr Baker said. The first was acquisition, or how likely a person was to contract the virus in the first place.
“We know that vaccines are effective at preventing infection, although the estimates of vaccine effectiveness depend on the vaccine type, time since dose and number of doses,” Dr Baker said in an email. Without contracting the virus, it was impossible to pass it on to anybody else, he said.
Various studies have shown that COVID-19 vaccines reduce the risk of infection, however research also indicates their effectiveness wanes over time – particularly against the Delta variant (see here, here and here).
The second aspect of contagion was the likelihood of someone transmitting the virus if they become infected. Multiple studies have shown that infected vaccinated people are less likely than infected unvaccinated people to pass on the virus.
An October 2021 preprint study from the Netherlands found that fully vaccinated participants with COVID-19 were significantly less likely than unvaccinated participants to spread the virus to unvaccinated members of their household.
After adjusting for age differences between participants, the researchers concluded that the effectiveness of full vaccination against transmission to unvaccinated household contacts was 63 per cent. The study used data from August and September 2021, by which time the Delta variant was well established as the dominant strain of the virus.
August 2021 research, supported by Public Health England, found the likelihood of household transmission was approximately 40 to 50 per cent lower when index patients had been vaccinated 21 days or more before testing positive when compared to unvaccinated index patients. However, the data used in the study dated from early 2021, before the more infectious Delta variant had taken root.
An October 2021 study of 814,806 families in Sweden – which also relies on cases recorded prior to the dominance of the Delta strain – found the risk of unvaccinated family members without immunity from previous infection contracting COVID-19 dropped by at least 45 per cent when they lived with one immune family member, and up to 97 per cent when all other members of the household were immune.
It said the benefits were similar regardless of whether the immunity had been acquired through previous SARS-CoV-2 infection, a single vaccine dose or full vaccination.
Other studies (see here and here) have similarly shown that vaccination reduces the chance of an infected person transmitting the virus, including UK research involving a large cohort of healthcare workers.
The Lancet study referred to by Ms Hanson may be UK research published in October 2021, which found that vaccinated and unvaccinated people transmitted the Delta strain to household contacts at similar rates.
The study, which was led by researchers from Imperial College London (ICL), concluded that although vaccines “remain highly effective at preventing severe disease and deaths” from COVID-19, “vaccination is not sufficient to prevent transmission of the Delta variant in household settings with prolonged exposures”.
It has been reported as evidence that vaccines do not reduce the risk of passing on the virus within a household, however the authors of the study have cautioned against making such an interpretation.
Explaining their findings in an ICL article, the researchers said that unvaccinated participants in the study were on average younger than the vaccinated participants, and younger people were known to be less infectious than older adults.
“The overall conclusion is therefore that vaccinated breakthrough cases can efficiently transmit infection but most likely at a lower rate than age-matched unvaccinated persons,” they said.
Several studies have indicated the peak viral load among vaccinated and unvaccinated COVID-19 cases are similar (see here, here, here and here), but this does not mean that both groups carry the same risks of transmission.
Dr Baker said while recent studies about viral load may appear to contradict evidence that vaccinations reduce the chance of viral spread, in reality the relationship between viral load and transmission was “a complicated picture”.
As explained by immunology experts at Victoria University in a November 18 article in The Conversation, vaccinated people cleared the virus faster, had a lower overall viral load and were therefore less contagious than the unvaccinated, despite carrying similar peak viral loads.
“Given vaccines speed the clearance of COVID from the body, vaccinated people have less opportunity to spread the virus overall. This appears to be the case even with the more infectious Delta variant,” they said.
The Verdict
Vaccinated people are less likely to become infected with the virus that causes COVID-19 than the unvaccinated, and if infected are less likely to transmit the coronavirus to others, research shows. The combination of these factors means a vaccinated person is, on average, far less likely to spread the virus than an unvaccinated person.
False – The claim is inaccurate.
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