On June 25, Wales became the first UK nation to completely vaccinate 50 per cent of its total population, the latest benchmark in a vaccination program which has become regarded as one of the best in the world.

The latest data from Public Health Wales shows that 2.26 million people in Wales, approximately 72 per cent of the population, have now received a first dose of the vaccine, a higher proportion than any other country. It is a remarkable achievement which scientists attribute to local vaccination centres offering the jab to as many people, as quickly as possible.

“There was a drive to get single doses out, so there was partial protection in a wider range of the population,” says Michael Gravenor, an epidemiology professor at Swansea University. “So instead of saying we vaccinate all over 70s, then all over 60s, and agonising over the diminishing returns in a certain age group, we got into the lower bands quite early in the rollout. And recently there have been campaigns to boost vaccine uptake and awareness in younger populations.”

But even with this widespread vaccination coverage, cases of Covid-19 across Wales are doubling every seven to ten days, as the third wave takes hold. One of the problems is that the emergence of the Delta variant has made achieving some form of herd immunity far more difficult to reach. A quick reminder: herd immunity is when enough people in a population acquire immunity either through vaccination or infection that new infections no longer spark big outbreaks. It doesn’t mean that we’d see no new infections at all, but it does mean an end to the runaway train of infections that has been a hallmark of the pandemic so far.

In the most simplistic terms, epidemiologists calculate the threshold for achieving herd immunity based on the R value, the average number of people that an infected person can transmit the virus onto. While the original Sars-CoV-2 strain had an R value of between two and three, the Delta variant is far more transmissible although current estimates vary. Some scientists have estimated Delta to have an R value of seven, while Karl Friston, a University College London professor and Independent SAGE advisor who has been modelling Covid-19 since the beginning of the pandemic, estimates that it’s slightly lower than that. “Our analyses suggest that the R value is about five at this point in the year, and that's an important qualification because of seasonal fluctuations,” he says.

According to Gravenor and Friston, an R value between five and seven means that somewhere between 80 and 85 per cent of the population need to be protected in order to achieve herd immunity, a considerable jump from the original estimates of 60-70 per cent, early in the pandemic. At face value, this would no longer appear viable given that current government policies do not involve vaccinating children. Even in the unlikely event of every single adult in the UK receiving the vaccine, this would still only represent 78 per cent of the UK’s 66.7 million population.

There have been hopes that this figure may be boosted by natural immunity, for example among the under 18s who have acquired protective antibodies through already being infected with the virus. “There is certainly a lot of pressure on the attainability of the herd immunity threshold due to the Delta variant, but it doesn't necessarily mean that it is out of reach,” Friston says.

However as we’ve learned more about how our immune systems react to the Delta variant, achieving the herd immunity threshold has become more and more unlikely. While data from the Office for National Statistics shows that eight out of ten adults in England, Wales and Northern Ireland now have antibodies against Sars-CoV-2, either through previous exposure or vaccination, merely having antibodies is not enough to stop the virus in its tracks.

It is already known that Sars-CoV-2 can continue to spread among partially-vaccinated individuals – although the risk is much lower than in unvaccinated people – but in recent weeks, new information has appeared which suggests that Delta is still capable of being transmitted by those who have had both doses. Doctors at Clalit Health Services, Israel’s largest health care organisation, found that around half of adults infected in a new outbreak of Delta had been fully inoculated with the Pfizer vaccine. This news has prompted Israel to re-impose an indoor mask requirement and reassess its Covid-19 regulations.

At the same time, a study released by a team of infectious diseases specialists at the University of Cambridge has found antibodies in blood samples taken from individuals who have received two doses of the AstraZeneca vaccine were much less responsive to the Delta variant than the original strain of Sars-Cov-2. This is thought to explain how the virus has continued to spread among healthcare workers in hospitals across India, even though they have been fully vaccinated with the AstraZeneca jab. One report in the eastern state of Odisha, reported 274 cases in fully-vaccinated healthcare workers between March 1 and June 10.

Combined with the fact that antibody responses may wane over time, both in people who were infected by the virus and vaccinated individuals, Gravenor suspects that achieving herd immunity is now an unrealistic goal. While the UK vaccination campaign has been successful, the majority of other countries are very far from vaccinating their populations with two doses, meaning that the virus will continue to be reintroduced from other parts of the globe as borders open and travel restrictions are released.

“If we're in a situation of a reasonable proportion of vaccinated people being able to transmit the virus, then it's very hard to see how onward transmission won’t continue,” he says.

However even if herd immunity is out of reach, that may not be a major problem. Gravenor points out that even if the virus is spreading, this is less of a problem if it is not causing severe illness, and hospitalisations remain low. “The disease can go away before the virus goes away,” he says. “Because they're two different things, so we could talk about levels of this disease that are very low, even if there is still a certain amount of transmission.”

This is not to say that scientists are not still worried about the ongoing spread of Delta. More transmission provides the virus with more opportunities to evolve, and the potential consequences of that are difficult to predict. But in the meantime, epidemiologists are looking to conduct more complex modelling of how exactly the virus is passing from person to person, taking into account the various degrees of immunity against it, and also the structures of different communities within the population.

“We’re seeing reports of vaccinated people who then test positive, but that could be quite a low level of infection,” says Gravenor. “The extent to which they then pass on the virus is still difficult to ascertain, I think it'll be a while before we know how much transmission is stopped by vaccination.”

Gravenor thinks that as the UK gets closer and closer to completing its vaccine rollout, we will reach a point where different clusters of infections do continually emerge from time to time within localised communities, but there will no longer be the mass waves we have seen over the past year.

“Variabilities between communities will be more relevant in terms of whether an infection will take off or not,” he says. “But in a country with high vaccination, larger outbreaks will be unlikely because even if cases are imported, they might spread a little bit within a certain area, but the overall R value has been brought right down.”

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This article was originally published by WIRED UK