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FactCheck: Are vaccinated people as likely to transmit Covid-19 as non-vaccinated people?

Did a study show a vaccinated person is every bit as likely to transmit Covid as an unvaccinated person? We test a claim made in the Dáil by Wexford TD Verona Murphy.

 

For Covid factchecks (5)

A TD MADE a claim in the Dáil this week about Covid transmission which led to the Minister for Health criticising the spread of ‘anti-vax information’ by politicians. 

It happened during a debate on Tuesday evening over legislation that would extend the government’s powers to enact Covid-19 restrictions.

Independent Wexford TD Verona Murphy told the Dáil: 

“There has yet to be any evidence presented which shows that vaccine passports actually serve any purpose in preventing transmission. In fact, recent studies have shown that a vaccinated person is every bit as likely to transmit this virus as a non-vaccinated person… therefore why is the government continuing to divide society on this basis?” 

She also said, “The constant scapegoating of the unvaccinated does not stand up to scrutiny or evidence and is completely unwarranted.” 

Things got heated in the Dáil chamber when Health Minister Stephen Donnelly loudly interjected across the chamber floor: “That information is false and it’s really important that members of parliament not spread anti-vax information.”

Deputy Murphy rejected the assertion, later telling Pat Kenny on Newstalk on Wednesday: “Let me be clear: I am not anti-vax.” She claims she was unable to finish her speech due to the interruption. 

Parliamentary tensions aside, was Deputy Murphy’s claim correct that recent studies have shown that vaccinated people are just as likely to transmit Covid-19 as unvaccinated people? 

What claim are we checking?

‘Recent studies have shown that a vaccinated person is every bit as likely to transmit this virus as a non-vaccinated person.’  

Who said it?

Independent Wexford TD Verona Murphy 

The evidence:

Murphy confirmed during an interview on Newstalk that she based her claim on a study published in The Lancet Infectious Diseases, a respected peer-reviewed medical science journal, with the catchy title: ‘Community transmission and viral load kinetics of the SARS-CoV-2 delta (B.1.617.2) variant in vaccinated and unvaccinated individuals in the UK: a prospective, longitudinal, cohort study.’

The Imperial College London study tracked transmission and viral loads in vaccinated and unvaccinated people by recruiting positive cases from the UK contract tracing system and testing the people they live with for 20 days to see how the virus spread.  

Among other things, the study found that fully vaccinated people who had tested positive for Covid-19 had a peak viral load similar to unvaccinated people, and could transmit the infection in household settings, including to fully vaccinated close contacts. 

It found that when it came to secondary transmission (someone else in the household getting Covid), the attack rate – the likelihood of people getting infected – when someone in the house was exposed to a vaccinated positive case was 25%. That’s slightly higher than the attack rate of when exposed to an unvaccinated positive case which was 23%. 

So does this mean this is definitive proof that vaccinated people might be as every bit as likely to transmit the virus?

Well, not exactly

The study showed that both unvaccinated and vaccinated people have a similar peak viral load. Viral load is the number of viral particles in your body taking over cells to multiply itself. The more of these that exist, the more likely you are to transmit the disease to someone else.

While vaccinated people are as infectious at their peak as unvaccinated people, it is understood they spend less time at their peak than unvaccinated people do. This means overall there’s a lesser chance of vaccinated people passing on the virus because they spend less time at their peak infectiousness. 

Dr Simon Clarke, a microbiologist from the University of Reading, told The Journal

“Fully vaccinated people cleared the infection more quickly than those who are unvaccinated, but their peak viral load was similar to that seen in unvaccinated people, which may explain why they can still readily pass on the virus in household settings.

“The peak viral load is roughly the same between both but that peak lasts slightly shorter in vaccinated people compared to the unvaccinated shedding a maximum amount over a longer time.”

When put to him, Dr Clarke rated Verona Murphy’s claim as having a ‘partial truth’ to it but added: 

“Ultimately someone who has been vaccinated is less of a risk of passing the virus on than someone who hasn’t been. Contact with someone who has been vaccinated is less likely to result in a transmission than someone who is not.” 

Paul Moynagh, Professor of Immunology at Maynooth University, explained how the study’s household setting needed to be taken into account when looking at the results:

“What this shows is that in household transmission, they [the researchers] looked at secondary attack rate. The vaccinated had the secondary attack rate of 25% but the unvaccinated had a 38% attack rate. In vaccinated people, as the virus clears more quickly, there’s a lesser chance to infect people – but in households that is minimised because you’re in constant contact, sharing a house everyday with these people.”

In Tuesday’s NPHET briefing, Dr Cillian De Gascun echoed what these scientists said. He noted that while the viral load between the unjabbed and jabbed might be equivalent, “what’s interesting is that… the rate of incline, the rate of growth in the vaccinated individuals will be slower, and the rate of decline is probably greater.

“So even though they have an equivalent peak viral load, they’re probably responsible for fewer retransmissions, because they’re probably infectious for a shorter period of time.”

What the study’s authors say:

For one last final check we went back to the source of the research. We ran Verona Murphy’s claim by the authors of the Imperial College of London study that she got her facts from.

Chair of Infectious Diseases Professor Ajit Lalvani confirmed to The Journal that while the vaccinated population still has a high risk of transmitting the virus, the claim ‘a vaccinated person is every bit as likely to transmit the virus as the unvaccinated’ is incorrect. 

Professor Lalvani said vaccination reduces transmission in two ways: “First, by reducing the number of people who get infected. Second, by reducing the infectiousness of those who do get infected (vaccine breakthrough cases).

“Our data is consistent with vaccination having both these effects, but probably to a lesser extent than previously believed due to the greater infectiousness of the Delta variant.”

In addition to the Delta variant, age of participants might have also impacted the results. 

According to Professor Lalvani, the study’s data on transmission skewed quite young because the study took place in the UK where the unvaccinated are mostly kids and teenagers.

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“Since these younger age groups are known to be somewhat less infectious than older adults, the conclusion is therefore that vaccinated breakthrough cases can efficiently transmit infection in households but most likely at a lower rate than age-matched unvaccinated persons.

“[We] need to compare groups of the same age range, which was not possible because most UK adults are vaccinated and most UK teenagers unvaccinated.”

The study’s sample sizes were ‘small’ according to Dr Lalvani.

“This means our estimates (transmission) are not precise. The transmission from vaccinated cases could therefore be as low as 15% and that from unvaccinated cases as high as 31%, which is consistent with other research which suggests that vaccinated cases are half as infectious as unvaccinated cases.”

Lastly, he confirmed that the study was restricted only to an intimate household setting so the results can’t be used to confirm a wide-ranging statement like ‘the vaccinated are just as likely to pass on the virus as the unvaccinated’. 

“Our findings are specific to households or similar indoor settings with close and prolonged exposure. It would be wrong to infer that vaccinated Covid-19 cases are equally infectious outside the household setting.”

Verdict:

We rated this Mostly False.

The claim contained an element of truth – vaccinated and unvaccinated people in the study showed a similar viral load. At their peak viral load, they can both carry a similar risk of transmitting the virus, particularly the Delta variant. But unvaccinated people spend a longer time at that peak than the vaccinated, making them potentially infectious for a longer amount of time.

This combined with the study’s household setting, the lower secondary attack rate and other factors means it is mostly false to claim ‘a vaccinated person is every bit as likely to transmit this virus as a non-vaccinated person’. 

The study’s results were complex and led to headlines like this one in the Guardian:

“Jabs do not reduce risk of passing Covid within household, study suggests”

It’s easy to see how messages became mixed around the findings.

If Deputy Murphy had claimed ‘a recent study had shown the vaccinated had similar viral loads to those of the unvaccinated’ then she would have been correct. But she didn’t. The broad claim that the vaccinated are every bit as likely as the unvaccinated to transmit the virus is incorrect and on the balance of available evidence we are rating it Mostly False.

 

This article was edited on 19 November to correct a sentence which gave inaccurate statistics for the likelihood of someone getting infected from a household contact. The correct figures are 25% for the secondary attack rate in fully vaccinated household contacts and 23% in unvaccinated contacts.

TheJournal.ie’s FactCheck is a signatory to the International Fact-Checking Network’s Code of Principles. You can read it here. For information on how FactCheck works, what the verdicts mean, and how you can take part, check out our Reader’s Guide here. You can read about the team of editors and reporters who work on the factchecks here.

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