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A dark winter ahead?

The NHS faces a tough winter. Already under pressure from the backlog of cases that built up during the pandemic, the spectre of a COVID resurgence, along with the usual seasonal flu, threatens to overwhelm services.

A vital step towards relieving some of that pressure would be to understand better what effect the vaccination programme is having on the spread of COVID – a situation that became a little clearer with the recent publication in

The Lancet Infectious Diseases of a paper called “Community transmission and viral load kinetics of the SARS-CoV-2 delta variant in vaccinated and unvaccinated individuals in the UK: a prospective, longitudinal, cohort study”.

Viral load trajectories

Taking data from the community-based Assessment of Transmission and Contagiousness of COVID-19 in Contacts study, a team drawn largely from Imperial College London and Public Health England looked at over 8000 samples from 471 index cases of COVID between September 2020 and September 2021. In particular, the researchers compared the viral load trajectories of the delta variant in people who were fully vaccinated with those who remained unvaccinated and how this might affect the way the virus spreads within households.

They found that while those who had received both vaccinations were less likely to become infected with the delta variant, their risk was still significant. Once infected, they were almost as likely to pass the infection on to other people, especially in close-contact environments such as households.

Mild infections

The study co-lead, Jake Dunning, an honorary senior clinical lecturer at Imperial College London, says that a key facet of the study is that it concentrated on mild infections in the community. “That’s where transmission mostly occurs, particularly in households. Most of the participants in our study were exposed to the virus within a household and became infected there. The other thing to say is that the primary strategic aim of vaccination has always been to prevent severe disease and deaths. When the COVID vaccines were launched I think it came across that they were also going to prevent all infections, and while they are undeniably effective, this was always going to be the weaker effect.”

The study began life as a simple investigation of the secondary attack rate in community transmission. “So how many people exposed to the virus actually show evidence of infection? And what’s the earliest day following exposure that evidence becomes apparent?” says Dunning. “We collected data on symptoms, took daily swabs and ran lab tests to detect the virus. It sounds like an obvious thing to do, but was actually only being done for specific groups at the time, such as healthcare workers or professional sports people.”

The study evolves

The work proceeded as planned until everything began to change. “The alpha variant emerged, the vaccines became available, then we had the delta variant. Suddenly we had the opportunity, via the daily sampling, to address other important  questions. So the study evolved with the pandemic, and the research questions adapted to meet the changing needs of public health.”

At this point, with the emergence of the more transmissible delta variant, the study shifted focus to look at the effect of the vaccinations on protecting people against infection. Asking such a question in the middle of a pandemic, when so much hope had been pinned on the vaccines, inevitably ran the risk of giving unpalatable answers. “Scientists would always say that you have to know the good and the bad, otherwise how do you ever improve?” says Dunning. “How do you know if your effectiveness against infection is less than it was previously? Our study has shown that the effectiveness of the vaccination against transmission is not 100%. But then, most experts would have told you it was never going to hit that level. Even in the light of that, I’m not sure anybody would have expected that we would have seen similar rates of infection in contacts exposed to vaccinated index cases with delta as in contacts exposed to unvaccinated index cases, with around one quarter developing infection in each group. We were also surprised by the examples of transmission between fully vaccinated index cases and their contacts, although thankfully there are still a minority.”

The positivity surrounding vaccines was reinforced by the necessarily simple messaging that encouraged people to book their jabs, though this meant some of the more nuanced aspects got lost.

“Once you start looking at infection against severe disease and death, that’s a harder message for people to understand. It’s a complicated situation. But the big message from our study is that while we have seen a reduction in effectiveness of the vaccines against infection, they remain a good way to prevent transmission of the virus. Importantly, they continue to reduce the risk of serious illness and prevent hospitalisation, even with the delta variant.”


Dr Jake Dunning MBE

  • 2001: Graduated with honours, Imperial College School of Medicine
  • 2009–2013: PhD in viral immunology, Imperial College London
  • 2014: Consultant Physician, Infectious Diseases (currently at the Royal Free Hospital, London)
  • 2014–2015: Lead Clinician and Co-investigator, Rapid Assessment of Potential Interventions and Drugs for Ebola clinical trials, West Africa
  • 2015–2021: Consultant at Public Health England (respiratory and enteric viruses, emerging infections and zoonoses, and COVID-19)
  • 2021: Senior Research Fellow, Epidemic diseases Research Group Oxford (ERGO), University of Oxford

Going into winter

On a public health front this winter, one of the key debates will centre on case counting and transmission.

“We should be thinking more about the difference between consequential infection – infection that leads to serious consequences – and just infection in itself, which is often not serious,” says Dunning. “But the tricky thing here is that, at a population level, even simple infections pose a threat to those who remain vulnerable to more severe illness.”

Going into this winter, what lessons might the health services draw from the study? “The discussion has started on whether to continue to count all cases of infection or focus on clinically significant infections. But if you don’t count on all fronts, you may miss early warning signals that increases in hospitalisations and deaths may follow. The lesson, I think, is to expect transmission to occur, even with high rates of vaccination. We’ve shown that. But it’s less clear whether it will lead to a huge increase in severe disease or death. Even that’s being tempered by the data coming out of Israel, the UK and elsewhere that people in high-risk groups who were vaccinated some time ago have waning of protection against infection and severe disease. That’s why we need the boosters and why the other non-clinical measures, as long as they are evidence-based and practical, should be used as part of a package.”  

Image credit | Getty

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