Why are Covid cases rising among double vaccinated?

By Jamie Hartmann-Boyce for The Conversation,

Sir Patrick Vallance, the UK’s chief scientific adviser, has announced that 40 per cent of people admitted to hospital with Covid in the UK have had two doses of a coronavirus vaccine.

At first glance, this rings very serious alarm bells, but it shouldn’t. The vaccines are still working very well.

There are several factors at play that explain why such a high proportion of cases are in the fully vaccinated.

Covid vaccines are extremely effective, but none 100 per cent so. This itself isn’t surprising – flu vaccines aren’t 100 per cent effective either.

Yet in the US alone flu vaccines are estimated to prevent millions of cases of illness, tens of thousands of hospitalisations and thousands of deaths every year. The Covid vaccines are doing the same in the UK right now – all one has to do is compare the curves from the winter wave with those from this summer.

As cases are rising, hospitalisations and deaths are rising too, but not at anywhere near the same level as they were in the winter. In the second half of December 2020 – a time when UK case rates were similar to what they are now – about 3,800 people were being admitted to hospital with Covid each day.

The average now is around 700. So though that’s still higher than we wish it was, it’s a lot lower than it was the last time we had this many infections.

Covid is also growing among the vaccinated because the number of people in the UK who have had both doses is continuing to rise. At the time of writing, 88 per cent of UK adults have had a first dose and 69 per cent a second. As more and more of the population is vaccinated, the relative proportion of those with Covid who have had both jabs will rise.

If you imagine a hypothetical scenario in which 100 per cent of the population is double vaccinated, then 100 per cent of people with Covid, and in hospital with Covid, will also have had both jabs. As with deaths, this doesn’t mean the vaccine isn’t working. It just means the vaccine rollout is going very well.

Also read: WHO warns of 'long term' Covid impact on mental health

We also need to remember that the vaccine rollout in the UK has systematically targeted people at the highest risk from Covid.

Older people and people with health conditions that make them more vulnerable were the first to get vaccinated. Once vaccinated, these people (including me) are at much lower risk from Covid than they would have been otherwise – but they are still at risk.

That means that when we compare people with both vaccinations being hospitalised to those who haven’t had both doses, we aren’t comparing like with like. People with both vaccinations are more likely to have been at greater risk from Covid in the first place. This makes them both more likely to be hospitalised and more likely to have already received both of their vaccine doses.

Is Covid different in the vaccinated?

The latest data from Public Health England suggests that against the delta variant, which is now dominant in UK, two doses of any of the vaccines available in Britain are estimated to offer 79 per cent protection against symptomatic Covid and 96 per cent protection against hospitalisation.

We don’t have clear estimates yet from Public Health England on the level of protection against death caused by the delta variant – fortunately, this is partly driven by the fact deaths have been relatively low during this third wave in the UK.

But for the alpha variant, Public Health England data estimates the Pfizer vaccine to be between 95 per cent and 99 per cent effective at preventing death from Covid-19, with the AstraZeneca vaccine estimated to be between 75 per cent and 99 per cent effective. The evidence we have so far doesn’t suggest that the delta variant substantially changes this picture.

There’s lots we still need to learn about how people with both vaccine doses respond to getting infected with the virus. The UK’s Covid Symptom Study is looking at this.

One of the key questions that remain is who is at most risk. Emerging data – released in a preprint, so yet to be reviewed by other scientists – suggests people who are overweight or obese, poorer people, and people with health conditions causing frailty seem to be more likely to get infected after having both jabs.

The preprint also suggests that age itself doesn’t seem to affect chances of developing Covid after being vaccinated, nor does having a long-term condition such as asthma, diabetes or heart disease – but we need more data on this to be sure of these findings.

Generally, the Covid Symptom Study has found that people report the same Covid symptoms whether or not they’ve been vaccinated, but that people who’ve been vaccinated have fewer symptoms over a shorter period of time, suggesting less serious illness. The most commonly reported symptoms in people who had had both doses were headache, runny nose, sneezing, sore throat and loss of smell.

(The author is a Senior Research Fellow, Departmental Lecturer and Director of Evidence-Based Healthcare DPhil Programme, Centre for Evidence-Based Medicine, University of Oxford)