Omicron and hospitalisation: new data on the protection from vaccines

Released yesterday (22nd December) are data from a study which has examined the rate of hospitalisation following infection with Omicron addressing the questions (i) does it differ from the rate following Delta infection and (ii) what is the influence of vaccination on this rate

The study

In this very rapidly produced report* (data was collected as late as 21st), researchers at Imperial College have undertaken a preliminary analysis using nationally available data for England


What data did they use?

  • Linked 3 national datasets which were:
  1. All patients with a positive PCR positive, up to December 14th, for one of the two variants identified: either Delta or Omicron
  2. All admission to NHS hospital (up to 21st December) with CoVid
  3. National Covid-19 immunisation database of CoVid vaccine status

What analysis did they do?

  • Compared hospitalisation rates in patients who were diagnosed with an infection between those with Delta and those with Omicron
  • Separate analysis by vaccine status: both by number of doses (1, 2 and 2+booster) and whether the vaccine for the first two doses was Pfizer/Moderna or AstraZeneca
  • The analysis took account of  age, gender, ethnicity and socio-economic status that might have influenced hospital admission 

What did they find: overall hospitalisation

  • The study identified around 55,000 patients with a positive PCR for Omicron, of whom approximately just over 200 were admitted to hospital: a rate of 4/1000
  • This rate of admission was about 10% lower than the rate of admissions following Delta infection after allowing for all the factors mentioned above

What did they find: hospitalisation by vaccine status?

  • I have extracted the data by vaccine status following (i) 2 doses and (ii) 2 doses plus Pfizer booster and did this for both if the first two doses were AstraZeneca and if the first 2 doses were Pfizer or Moderna
  • The data below show the percentage reduction of being admitted to hospital with (i) Delta or (ii) Omicron by vaccine status compared with the risk of being admitted in the unvaccinated 
  • Let me explain: 
    • The first blue bar on the left, shows a rate of 62%.  This means that 2 doses of the AstraZeneca vaccine reduces the risk of admission to hospital following an infection with Delta by 62% compared to those people who had had no vaccine at all
    • The first orange bar on the left, shows a rate of 69%.  This means that 2 doses of the AstraZeneca vaccine reduces the risk of admission to hospital following an infection with Omicron  by 69% compared to those people who had had no vaccine at all
  • Thus the chart thus shows that  people who had had AstraZeneca  for their first two doses had a greater protection against admission to hospital with Omicron than with Delta.  
  • People who had had Pfizer/Moderna for their first two doses had more protection against both variants than the data from AsttraZeneca
  • By contrast with the AstraZeneca data there was only a slightly greater protection against admission to hospital with Omicron than with Delta 
  • The second chart below compares the hospital admission rates between these two variants in people who had also had a booster: again giving data separately for those whose first two doses were (i) AstraZeneca and (ii) Pfizer/Moderna
  • The results are broadly similar
    • There is no evidence that hospitalisation risk was higher with Omicron infection than with Delta
    • The opposite seems more the case: with a greater protection against admission following Omicron (78%) compared to Delta (64%) in those who had had AstraZeneca and a Pfizer booster
    • By contrast the rate of hospitalisation following Delta or Omicron are very similar in those who had had Pfizer/Moderna and a Pfizer booster
    • These data suggest that first two doses AstraZeneca plus booster possibly give a greater protection 

Data warning

  • These are very preliminary data and a longer time is needed to obtain more precise results
  • The numbers of people admitted to hospital following Omicron who had had 3 doses of any of these vaccine schedules is still low (around 30) which is good news; but means that the results, say from the comparison between the vaccines, may not hold up with larger numbers 

My headline conclusions

  • Firstly, admission to hospital following Omicron infection is very low.  This is likely to be explained in large part by most people in the population having protection from being vaccinated (and that many of those infected with Omicron are young) 
  • My second conclusion is that there is no evidence that hospitalisations were higher following Omicron than with Delta, if anything at this stage may be slightly less likely
  • This is important as there has been a lot of laboratory data suggesting that the mutations in Omicron could mean that the antibodies we had from the vaccines were both reduced in amount and could be less effective, compared to Delta.  This does not appear to be the case
  • It would be a wrong conclusion that there is no worry that Omicron infection will lead to hospitalisation in those who are vaccinated but more that the risk for individuals is the same independent of whether we are infected with Delta or Omicron 
  • The unexpected finding, which might not persist with larger numbers, is the apparent greater protection when first two doses of vaccine are with the AstraZeneca vaccine
  • The vaccine data have been confusing as most studies show a greater and more sustained benefit of Pfizer or Moderna after 2 doses compared to AstraZeneca
  • It is possible that the combination of 2 doses of AstraZeneca followed by a Pfizer  booster might give some additional benefit  over 3 doses of Pfizer, and this might just be more obvious with Omicron – we shall have to see
  • Overall, these data have to be reassuring given all the concerns from the laboratory data but they of course do not take away from the much greater infectivity of Omicron over Delta .

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5 replies on “Omicron and hospitalisation: new data on the protection from vaccines”

Alan, as a layman I follow at a distance but from what I read, Omicron is more transmissible
but milder in effect than Delta. This, according to a GP comment on this matter a few weeks ago, might mimic the 1918 flu virus in that ‘son of 1918’ was more transmissible but much milder than its parent and remains so today.


Dear Alan, thanks as ever for your views on these early figures – very helpful. Like many people my earlier doses were AZ and 6 months later I had a Moderna booster. It had seemed likely that this combination might be less effective overall, but perhaps the reverse applies. Maybe it’s a bit of good news when there is plenty of bad! It’s great to have the benefit of your scientific views in these uncertain times


Hi. I live in Israel where 4th vaccinations are now being given 4 months after the booster. Is this a case of over-vaccination? Would you recommend that people check their antibody levels before getting a 4th jab? Thanks


Yes, Israel has received a lot of publicity about their role out of a 4th dose. The evidence for this being beneficial is yet to be obtained from Israel or anywhere. My take on what experts are guessing (and it can only be a guess at the stage) is that there may be limited benefit from a 4th over a 3rd. The vaccines in use are still the original vaccine and some newer vaccines (like a second generation of Pfizer) will be available in the New Year for sure. Having said all of that there is probably no harm from a 4th dose but possibly a greater risk of side effects


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