New data showing vaccines still allow transmission of the virus means herd immunity may not be possible
Herd immunity “is not a possibility” because the delta variant can spread among vaccinated individuals, according to experts including the director of the Oxford Vaccine Group.
In a parliamentary panel on Tuesday, Professor Sir Andrew Pollard warned that herd immunity is a “mythical” concept and should not inform the design of vaccination programmes in the UK or across the globe.
“This virus is not measles – if you have 95 per cent of people vaccinated against measles, the virus cannot transmit in the population,” said Sir Andrew. “But we know very clearly with the coronavirus that the delta variant will still infect people who have been vaccinated, and that does mean that anyone who’s still unvaccinated, at some point, will meet the virus.”
He added that vaccines may slightly slow the transmission process, as there is some evidence that inoculated people are infectious for a shorter period of time, but warned “we don’t have anything” which will completely halt the spread of Covid-19.
Sir Andrew’s comments were echoed by several other experts offering evidence to the All-Party Parliamentary Group (APPG) on coronavirus.
Paul Hunter, a professor of medicine at the University of East Anglia, said herd immunity is now “unachievable” while Devi Sridhar, a professor of global public health at Edinburgh University, warned it is unlikely that we will hit a “magical threshold” where the spread of Covid-19 “disappears”.
The comments come amid a shifting understanding of the role vaccinations can play to reduce the circulation of Covid-19. There had been some hopes that, if coverage was high enough, the virus would no longer be able to spread – stopping the pandemic in its tracks and conferring some protection for those who are unable to have a shot.
Earlier this year, there were even suggestions that those most likely to spread Sars-Cov-2, including frontline workers and young people, should be vaccinated first.
But over the last fortnight, these hopes have unravelled. While vaccines have been shown to protect against severe illness, hospitalisation and deaths, mounting data suggests they do not halt transmission.
Mounting evidence that jabs don’t halt transmission
Data published last week by Public Health England found there is little difference in how much virus is present in vaccinated and unvaccinated people who test positive for Covid-19, suggesting the shots do not suppress viral replication as much as hoped. Scientists had believed that a lower viral load would prevent onward transmission.
The study chimes with evidence from the United States, where a recent study of an outbreak in Massachusetts found viral loads were similar among 127 fully vaccinated people and 84 others who were unvaccinated.
The research led the US Centres for Disease Control and Prevention (CDC) to reverse guidance that had stated fully vaccinated people do not need to wear a face mask indoors.
But other experts have stressed that, even with similar loads, vaccinated people are unlikely to transmit at the same rate as those unvaccinated – and more research is needed to confirm the research.
“Even if viral load may be the same, vaccinated people who become infected are less likely to be infectious than unvaccinated because vaccines reduce virus shedding time, symptomatic infection, and the presence of immune response will suppress the viable virus,” said Dr Muge Cevik, a clinical lecturer in infectious diseases and virology at St Andrew’s University.
However, the early findings have significant implications for the future of the pandemic.
It could make travel more difficult – as vaccination status alone may not be an indication of whether an individual could transport Covid to another country – and may also change the paradigm in discussions around whether to vaccinate children.
Guidance from the UK’s Joint Committee on Vaccinations and Immunisations currently says that 16-17-year-olds should be given Covid-19 jabs but, unlike a range of countries including the US and Germany, has stopped short of recommending younger children have shots.
“One of the strongest arguments that have been repeated is to vaccinate children to protect adults,” said Sir Andrew. “[But] vaccinating children is not going to completely block transmission, so it doesn’t achieve that goal.”
He added that, instead, leaders should use available vaccines to protect as many of the most vulnerable people across the globe as possible.
“Every dose makes a difference if it’s given to people whose lives might be lost this year,” he said. “We shouldn’t belittle the value of even a small donation of doses to countries where people may die from the virus.”
Francois Balloux, director of the Genetics Institute at University College London, added: “It is not so much anymore a ‘duty to others to get vaccinated but protection for oneself. There won’t be any ‘herd immunity wall’ to hide behind.
“There is an important fraction of the population who for all sorts of reasons are immunocompromised. “For the vast majority of them, they still benefit from being vaccinated. Some won’t. Widespread vaccination reduces the risk of exposure to them, but only very marginally so.”