It is the key question puzzling scientists and commentators – how is Britain managing to weather the latest deadly Covid storm while our European neighbours’ defences are crumbling?
On Tuesday, Pascal Soriot, the chief executive of AstraZeneca, offered a possible answer. He suggested that the durability of T-cells from the Oxford/AstraZeneca jab could be keeping older people safer than those on the continent, preventing hospitilisations and deaths even though virus cases are still high.
Several countries, including France, Germany, Spain and Belgium, restricted the AstraZeneca vaccine to under-65s in the early stages of their rollouts, claiming there was not enough data to prove the jab worked for older people.
That might have been an error, Dr Soriot suggested, because data is showing that the AstraZeneca jab offers long-term protection, hinting that its response is more durable than that from mRNA jabs such as Moderna and Pfizer.
The problem is that nobody has seen this data – and AstraZeneca could not say when it would be published.
Undoubtedly, T-cells play an important role in ongoing protection. Unlike antibodies, which wane over time, they offer long-term surveillance of past viruses, standing poised to trigger powerful immune responses as well as directly destroying infected cells. They also seem to work against new variants.
Almost half of the UK’s vaccine dosages have been AstraZeneca
Early data did suggest AstraZeneca was better at triggering T-cell immunity than Pfizer. In April, the University of Birmingham found that, five weeks after a first dose, people aged over 80 given AstraZeneca or Pfizer showed similar antibody responses – but double in the AstraZeneca arm had T-cell responses. The strength of the T-cell response was also three times as high than for Pfizer.
By mid-June, Oxford University researchers had found there was still a robust T-cell response from the AstraZeneca jab at six months, even after a single dose and despite big drops in antibody immunity.
Eleanor Riley, professor of immunology and infectious disease at the University of Edinburgh, said: "It seems that the Oxford/AZ vaccine initially induces higher levels of T-cells than the mRNA vaccines.
"These T-cells are important for long-term immune memory and also for inhibiting virus replication and killing infected cells once an infection becomes established.
"Since the AZ vaccine is slightly better at inducing these T-cells, the implication is that it may provide longer-term protection against hospitalisation and death. I think this is the point that Dr Soriot was making."
In July, scientists from Oxford and Switzerland also suggested long-term immunity could be greater for adenovirus vaccines such as AstraZeneca because they created "cellular training camps" for T-cells. Such vaccines are able to get into long-lived tissue cells which turn into T-cell factories, pumping out immune cells – perhaps for a lifetime.
Some experts believe that if AstraZeneca is providing a better T-cell response it might explain why Britain has recently suffered high infection rates but low hospitilisations and deaths.
Modelled data for Covid hospitalisations
Dr Peter English, a retired consultant in communicable disease control and a former editor of Vaccines in Practice, said: "It is plausible that this generated an excellent T-cell response, which means that while people can still be infected and infectious, they are unlikely to be seriously unwell."
Yet there is a sting in the tail of this hypothesis. Recent research has shown that, in the long term, mRNA vaccines also produce strong T-cell responses – maybe even stronger than AstraZeneca.
A paper by the University Hospital Tubingen in Germany, published in Nature on Tuesday, found that Pfizer produced nearly six times more T-cells than AstraZeneca 18 to 42 days after the second vaccination.
Real-world effectiveness data has also consistently shown that the mRNA vaccines are slightly better at preventing infection and serious disease than the AstraZeneca jab, which is inconsistent with claims that the Oxford vaccine is providing better protection.
What is more likely is that Britain has had so much infection recently that Covid is on the brink of becoming an endemic virus. The UK also got its booster programme started earlier, with more than 12 million adults now having had a third shot.
The latest figures from the Office for National Statistics (ONS) show that antibody levels are rising again in older age groups after having started to decline in the summer, meaning older people in Britain have more protection than in recent months.
Which EU countries have suspended use of the AstraZeneca vaccine?
In Europe, the take-up is much slower, with many countries still administering the booster only to clinically vulnerable people.
Whether it does eventually turn out that AstraZeneca affords more long-term protection, it is clear that rolling it out to the most vulnerable has prevented tens of thousands of deaths – and it was remiss of Europe to keep it out of the arms of its pensioners.
There will be myriad reasons why Britain is doing better than Europe. AstraZeneca has certainly played its part.