Omicron: emerging data on transmission, severity and protection from vaccines

Since the start of the Omicron story, there has been an unprecedented focus by national authorities on collecting the data to answer the key questions. English data are now available and although there is much to be learnt and the analyses are very preliminary, there is information emerging about this variant

How quickly is Omicron spreading?

• As widely reported in the media, although the absolute number of known cases is low (as I write this perhaps under 2000 in the UK) the rate of increase is very high
• Case numbers are doubling perhaps every 3-5 days
• These are the official data released yesterday from England of the total number of cases for every variant since the start of the pandemic*
  • Each curve shows the growth in the total number of cases by the time (in days) since each variant had been first identified.
  • Just look at the green line: Omicron and the light purple line: Delta (which I have arrowed)
  • The Omicron curve is the steepest
  • From the steepness of this slope, ie the rate of increase of Omicron cases, statisticians are therefore predicting a rapid rise in the total number which could reach half a million within a month or so

*Note data emerging in past 2 hours suggest that the numbers might already be at the 2000

• The reason for this is that Omicron is more transmissible than even Delta: it was highly suspected this would be the case given the mutations
• One estimate is that this increase is around 3 times
• I have tried to illustrate this in my diagram below

• Imagine there are two people in your household: one with Delta and the other with Omicron.  
• The data from household contacts shows that you are 3.2 times more likely to catch infection from the Omicron than the Delta member
• It is for this reason that we are confident that Omicron will overtake Delta as the origin of the majority of new cases, perhaps as soon as the end of this year

Is infection with Omicron more severe?

• Thus far the data are reassuring
• I am not aware that any of the first 600 English cases infected with Omicron have been admitted to ICU but this will change
• However, most cases of Omicron as shown in the Figure below are in those under 50, with only a tiny number over 70.  This could of course (see below) be related to the success of boosters in protecting this age group
• The numbers are increasing daily so the figures may be out of date very quickly

• The data from South Africa are also reassuring.  The Omicron cases in South Africa are predominantly in younger people, but again thus far the cases are mild
• Indeed, the Omicron cases in South Africa in patients in hospital are all incidental, ie the patients were in hospital for another reason and the Omicron infection was picked up on routine Covid-19 testing

What do we know about re-infection risk?

• People who have had a previous infection will have some immunity from that, in addition to any provided from their vaccines
• We know that this immunity does not completely prevent a second infection with any of the variants and also wanes with time – hence everyone is recommended for vaccines whether or not they have had a previous infection
• It is of interest though to know if the risk of re-infection is higher with Omicron
• This is a very challenging question to answer but there are preliminary data from England looking at how often a new case of Omicron is a re-infection compared with a new case of Delta
• The current estimate is, for those with a previous definite infection, the likelihood of contacting a second infection due to Omicron is 5 times higher than that due to Delta
• On the one hand this is not surprising, given what we know about the difference between Omicron and Delta, but there are a number of issues in the way this increased rate was calculated and this figure could well be an overestimate 

What do we know about the effectiveness of current vaccines against Omicron?

• I make no apologies for emphasising that, although we have some data, these are very preliminary and the way the data have been derived could lead to the results looking worse
• The English data have looked at 56000 recent Delta cases with symptoms and compared their vaccine history with nearly 600 Omicron cases also with symptoms
• The results looked separately at the rate of cases (i) 6 months after 2 doses of vaccine and (ii) 2 weeks after a booster
• All boosters were with Pfizer but there was a separate analysis depending on whether the first 2 doses were AstraZeneca or Pfizer
• Here are the results of the percentage effectiveness for those who had AstraZeneca for their first 2 doses:

• The blue bars show the results for Delta and confirm that, as  we knew, the protection had waned by 6 months to around 40%.  In addition, there was very little protection against Omicron
• After the booster, there was a very good protection against Omicron of around 70%, but less than the 95% against Delta
• Here are the results of the percentage effectiveness for those who had Pfizer for their first 2 doses

• The protection after 2 doses of Pfizer was higher than for AZ against both Delta and Omicron, although for the latter was under 40%
• The data after a booster were the same as for AstraZeneca
• Note that these are the results from all symptomatic cases, most of whom were mild
• The rate of protection following a booster against severe cases could be higher
• How long the protection following a booster will last is of course unknown

Conclusion

• It is still early days and there is much we need to find out
• The data I have presented come with a number of ‘health warnings’ which epidemiologists have been keen to emphasise
• The conclusions may well change with more data and more sophisticated analysis
• When I look to see what I posted a year ago about the then new variants, I considered many of the same questions, although we did not have the vaccine coverage we have now

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