Despite Delta, severe covid-19 is much rarer among vaccinated Britons

CHINKS HAVE showed up of late in covid-19 vaccines’ armour. With the Delta variant now dominant and many people already seven months removed from their second shots, estimates in recent academic papers of protection rates against infection have ranged widely, from 39-88%.

On September 9th Public Health England (PHE), a British agency, released a report using a different approach from that of such studies. Rather than tracking trial participants or people selected on the basis of their medical histories, it follows England’s entire population from August 9th to September 5th, reflecting Delta-variant infections almost exclusively. The report measures the rates of covid-19 cases, hospitalisations and deaths for both vaccinated and unvaccinated people within the same age ranges, comparing groups that should be similarly vulnerable to the virus.

The most surprising result was that for people aged 40-79, cases appeared to be more common among the vaccinated than the unvaccinated. This may stem partly from PHE’s use of a health-services database that can double-count people who change addresses. If PHE overestimated the number of unjabbed people, it would underestimate the benefit from vaccines.

After PHE published its report, James Ward, a consultant, repeated its calculations using population estimates from the Office for National Statistics, which runs Britain’s census. This exercise reversed the pattern: cases became rarer for the jabbed than for the unjabbed below age 80, but more common above that age.

Unlike the murky case numbers, the data on hospitalisations and deaths were reassuring. Within age groups, these outcomes were 67-90% less common among the vaccinated than the unvaccinated—a result similar to the 90% decline found in data from America released on September 10th. PHE estimated that the jabs have prevented 112,000 deaths in Britain. The country’s official toll from covid-19 is 134,000.

PHE’s data offer two reasons to hope that Britain may avoid an autumn surge in severe covid-19. One is booster jabs, which begin this month. The gap in death rates between the vaccinated and unvaccinated was smallest among people aged over 79. Weak immune systems in this group could cause such a pattern. But another potential explanation is antibodies, the body’s first line of defence against familiar pathogens.

Antibodies wane over time. Because old people were the first to get shots, the average amount of time that has passed since they got their jabs is unusually long, meaning that their antibody levels may by now be rather low. When young vaccinated people with few antibodies are infected, they often expel the virus quickly, yielding mild cases. In contrast, some old people need high levels of antibody protection to avoid severe disease. And booster shots cause a surge in antibodies. In Israel admissions to hospital among older people who have received boosters have fallen sharply.

The second encouraging trend is that the novel coronavirus is no longer so novel. PHE also studied the prevalence of various antibody types among blood donors. Fully 98% had S antibodies, which appear in response either to the virus or to vaccines. Moreover, 18% also had N antibodies, which show up only after exposure to the virus. Among people aged 17-29, the least vaccinated group, this share was 28%, implying that natural immunity may be unusually common among the unjabbed.

New cases in Britain are near their January peak, but jabs still stop most serious disease. That merits cautious optimism.■

Sources: Public Health England; ONS