A combination of three factors – waning vaccine immunity, more social contacts and a new strain of Omicron – has led to a surge in COVID cases and an increase in the number of older people being admitted to hospital.
New real-world data from the UK Health Security Agency suggests that vaccine effectiveness is now waning for the 22 million most vulnerable people in the country who received their booster jab more than 15 weeks ago.
It reveals that people aged 65 and over are three times more likely to need intensive care if they get infected with Omicron 15 weeks or more after their third injection.
After that period, they are 86.8% less likely to need a ventilator than the unvaccinated, compared to 95.8% for those infected within 14 and 34 days of their booster.
This could be one reason why hospitalisations have risen beyond the January peak for those aged 65 and over, and why the government has this week begun a programme of fourth doses for the most vulnerable groups.
Read more:
COVID-19 cases in UK soar by around a million in a week
Omicron has been less likely to cause severe disease than other COVID variants, but as we’ve seen in countries like Hong Kong, which had poor levels of protection when Omicron hit, it can still cause a high number of deaths.
Three reasons for rise in hospitalisations
Professor John Edmunds, an epidemiologist at the London School of Hygiene and Tropical Medicine and an attendee at SAGE, says the vaccine waning effect is one of three key reasons why there’s been a recent rise of COVID patients in hospital.
The other two reasons are the prevalence of a more infectious “cousin of Omicron”, and an increase in mobility and social contacts since restrictions were loosened in England on 24 February.
The combination of these two factors has driven infections in England close to the record level set in January, rising by around one million to a total of 4.3 million people across the UK in the week up to 19 March.
Infections in Scotland are at a new high, with one in 11 people infected in the latest week.
The waning effectiveness of the vaccine now means that more of those cases are likely to need intensive health care.
The ‘cousin of Omicron’
The new strain of Omicron is officially known as BA.2, separating it from BA.1 which took hold in the UK in December.
It is now responsible for almost nine in 10 cases in England.
Professor Edmunds says “it is clearly more transmissible, which is quite a feat given that BA.1 was already extremely transmissible”.
The latest UKHSA data suggests that each BA.2 case leads to about 75% more cases than BA.1.
“From what we know at the moment, it’s no more likely to cause severe disease than BA.1, which is relatively good news.”
Hospitalisations caused by COVID v hospitalisations with COVID
When cases rise in the population at large, they also do in other settings, including hospitals.
While COVID hospitalisations are increasing, it doesn’t mean all those people have been admitted because of the virus.
Currently, just under half of people in hospital with COVID are there because of the virus.
Just because someone’s reason for being in hospital isn’t COVID, it doesn’t mean that their infection isn’t relevant to their treatment.
From a practical point of view it causes problems because NHS staff have to put in place special measures to avoid infection themselves, and to avoid the virus passing to other vulnerable patients.
That means their work is slower and it takes longer to get to other people waiting for treatment.
From a medical perspective it can cause further complications for people whose other health problems are already serious enough to take them to hospital.
Mobility and social contacts
When it comes to mobility and social contacts, it might seem that things haven’t changed too much in recent weeks, but Professor Edmunds says there has been “a small acceleration back towards something akin to normal” and that is potentially quite significant.
The number of people who had COVID-19 has risen by 76% since restrictions in England were lifted in February, according to ONS data up to 19 March.
“We are still a very long way from where we were two years ago,” said Professor Edmunds.
“For most people, the majority of your contacts pre-COVID would have been at work. You might see a certain number of friends on a Friday night and the people you live with every day at home, but work is responsible for a much higher number.”
Although people are still working from home in high numbers, a small change to that number can mean a bigger increase in the number of people you make contact with, explains Professor Edmunds.
For example, in an office of 10 people, if everyone comes in five days a week, all 10 people will have nine contacts per day, so 45 contacts per week.
If everyone comes in two days out of five, that means there is only going to be an average of four other people in the office with you. Three out of five people will be at home.
So while your mobility is reduced 60%, your contacts for that day have gone down two thirds – from nine to three.
And as you’re only doing that twice a week as well, your total work contacts for the week are six instead of 45.
If everyone went in three days a week, your contacts would go up to an average of six per day, or 18 a week. So going in three days a week rather than two means your contacts are multiplied three times.
“For society it is a good thing, and for the economy. But epidemiologically the extra contacts are a problem.”
Despite that, Professor Edmunds said that he doesn’t think there will be a need for future lockdowns.
“We will go to pharmaceutical measures first, like extending the vaccine programme. I don’t think there is the stomach for more lockdowns.”
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