Why the Delta variant has changed the race between vaccines and the virus
DELAYING a further lifting of Covid restrictions is about “buying time”, Nicola Sturgeon told MSPS.
The virus Scotland faces today is more dangerous on almost every measure than the one we were dealing with when the timetable for exiting lockdown was set out in March: the Delta variant is more transmissible, less responsive to vaccines, and doubles your likelihood of requiring hospital treatment.
It is already responsible for 75 per cent of Covid infections in Scotland (96% in England) and looks set to squeeze the previously dominant Alpha/kent strain close to extinction here by the end of June.
When we talk about “Covid”, what we are really talking about now is Delta – the B1.617.2 strain.
Everything we have learned about it so far helps explain why India, where it first emerged, descended into catastrophe so quickly.
For the UK, our exit from lockdown would probably have remained on track had we weeded out Delta cases early through Australian-style border measures requiring blanket hotel quarantine for all arrivals from January onwards.
On the other hand, were it not for vaccines we would almost certainly be facing a third wave of Covid deaths worse than anything we have experienced to date. So what will time buy us?
The key issue is more second doses – and more time to build up immunity from vaccinations.
According to research led by Edinburgh University, someone given a single dose of the Astrazeneca vaccine is only 18% less likely to become infected with the Delta variant (with or without experiencing symptoms) than someone who is unvaccinated – and that’s 28 days or more on from vaccination.
With the Pfizer vaccine (mainly given to under-40s, elderly care home residents, and frontline healthcare staff) , the protection is 30%.
Being fully vaccinated – with both doses – has never been more important.
The same research showed that protection against a Delta Covid infection rises to 79% (Pfizer) and 60% (Astrazeneca) two weeks on from a second dose.
Protection against hospitalisation was stronger still, according to Public Health England research: 96% with Pfizer and 92% with Astrazeneca.
Right now, 97% of over-60s in Scotland have had both doses. But one-third of people aged 50-54 and two-thirds of those in their 40s are still waiting for their second vaccination; these are the people being prioritised between now and July 19. In addition, half of Scots aged 18 to 39 still need a first dose.
The goal is that higher second dose coverage should mean less pressure on hospitals.
But a delay also buys us time to understand more clearly to what extent vaccines are weakening the link between Delta infections and serious illness.
Cases are doubling roughly every 16 days in Scotland. By June 30 we are on track to reach 1,800 cases per day – the same as in early January – and 3,600 per day by mid-july.
In May, 5% of infections were translating into a hospital admission. That is down from 10% in January, but were we to reach 180 hospital admissions per day by the end of July we would be on a par with the wave one peak (183 per day) and not far off the wave two peak (199 per day).
The good news is that vaccines mean hospital patients now are younger (mostly in their 30 or 40s), recover faster, are less likely to become critically ill, which should mean less pressure on intensive care and fewer Covid deaths.
As vaccines are rolled out that 5% ratio for cases-to-hospitalisations should also shrink – except that comparing people like-for-like (by age, by vaccination status, by morbidities etc) those infected with the Delta variant are 85% more likely to end up in hospital than those with the Alpha variant. Since Delta only overtook Alpha in mid-may, we are probably stuck with that 5% (at least) figure for a while yet.
Furthermore, the Delta variant appears to lead to roughly twice the rate of “vaccine fails”, meaning that even once the whole adult population is fully vaccinated we will probably end up with many more breakthrough infections – especially in the sick and elderly – than we would have done had Alpha remained dominant.
Its higher transmissibility (40-60% more than Alpha) also pushes up its R number and makes herd immunity harder to reach. If the original Wuhan strain had a natural R number of around three (the number of people each infected person passes the virus onto in the absence of any lockdown measures) we needed to vaccinate around 66% of the total population to stall the epidemic.
Delta has an estimated natural R of six, meaning we need to fully vaccinate at least 83% of the population. Given that 17% of Scotland’s population is under 16 and not all adults can or will be vaccinated, this is unlikely to be reached until immunisation is extended to 12 to 15-year-olds.