Double vaccinated with Pfizer or AstraZeneca: but how protected am I against new variants?

The initial euphoria following the roll out of vaccines is currently tempered by concerns that their effectiveness is less certain against new variants of the virus.  We now have data from both Pfizer and AstraZeneca vaccines but providing an answer to the question above is quite complex but hopefully this post will help!

New variants for old

  • The vaccines were developed based on the original ‘Wuhan’ strain of the virus and the first clinical trials, proving their success, were in populations exposed to that variant
  • It is obviously relevant to know if those early clinical trial results now also apply to the main current variants, especially the Delta (India) one

A quick comment on the new variants

  • Current focus is on the Delta variant, but there are a number of other variants that are beginning to emerge
  • The data are based on detailed DNA analysis of positive swabs
  • In the main the impact of the variants influences is on how infectious (transmissible) they are, rather than the severity of infection
  • The main reason I give this list with all the newer variants is not to generate anxiety but rather to emphasise that this growing number is an inevitable consequence of the evolution of new mutations during a pandemic
  • There are bound to be other named variants to come – so don’t even bother trying to memorise this list!
  • Although nothing in this pandemic is certain, our knowledge of how effective the vaccines are against the Alpha and Delta strains can probably also be applied to these newer variants 

Five questions to be asked

As someone who has been double vaccinated (with the AstraZeneca vaccine) I assess my risk by considering the following 5 questions:

  1. What is the rate of infection in the population I am exposed to?
  2. How transmissible are the current variants?
  3. How protected is the rest of the population I am exposed to?
  4. Will my vaccine stop me getting infected?
  5. If I do get infected, will my vaccine stop me getting seriously ill?

The answers are of course interconnected but I will deal with them in turn.

1.) What is the rate of infection in the population I am exposed to?

  • It is stating the obvious, but worth repeating, that if the infection rate is very low in my area, then it is less of a worry if my vaccine does not give perfect (100%) protection
  • In this map from last week in England, there was at least a 6-fold variation in the rate of infection between different regions 
To explain: a prevalence of 0.006 is the same as 0.6% of the population is infected
  • I’ve tried to illustrate the impact of this 6 fold variation in relation to my vaccine status in the diagram below, assuming that my vaccine gave me 90% protection. 
  • In an area with a high rate of infection there will be far more cases of infection in both vaccinated and unvaccinated people than in areas with low infection rates
  • The consequence is that the rate of infection in my area is (obviously)  important to my risk of being infected

2) How transmissible are the current variants?

  • Infection rates within a population do not stay static and thus I don’t only need to know what is the current rate of infection, but also is it changing?
  • The figure below comes from English data published this week
  • The top (very light!) grey/purple curve is the curve for Delta and that shows how much more transmissible this variant is than all the other variants analysed

  • Again, even if the vaccines were equally effective against all variants, looking to the short-term future, more vaccinated people will get infected in areas where the Delta variant predominates 

3) How protected is the rest of the population I am exposed to?

  • The hope from the start of the vaccination programme was that not only would I be protected by my own vaccine but also that people around me would have been vaccinated and they would be less likely to get infected and pass that benefit on to me (herd immunity)
  • To be honest it is still not clear how far this is the case.  As discussed below, the vaccines are not perfect against being infected
  • There are good data from populations looking at the trends in the numbers with infection as the vaccination rate increased:
    • For example, data from Israel and the elderly in the UK support that idea that high levels of vaccine uptake might also protect unvaccinated people in those populations
    • Such trends though do not prove cause and effect
  • My conclusion is that even if the vaccines were equally effective against all variants, although I would feel more protected if those around me were also vaccinated, I cannot quantify the change in the level of personal benefit that would bring

4) Will my vaccine stop me getting infected with the new variants?

  • We knew from the early clinical trials (i.e. before the new variants) that the vaccines did stop many people getting infected, but that this was not 100% 
  • Thus these clinical trials suggested that the short term protection was less than perfect for all the vaccines
  • The level of protection was also less with the AstraZeneca than with the Pfizer or Moderna vaccines 
  • We now have data, not from clinical trials but from much larger national population databases, on the effectiveness of the vaccines against the new variants
  • Both England and Scotland have provided such data on this issue
  • They linked:
    • Results from the national testing services which collected information on who tested positive and whether they had symptoms.  These data also gave information on which variant
    • The vaccine status, including data and number of doses and which vaccine 
  • By analysing the data from all the positive tests, they have been able to answer the following question: 
    • How much lower was the rate of infection with symptoms in people who had been vaccinated compared to people who had not been vaccinated?
    • And in particular
      • Was there any difference between AstraZeneca and Pfizer vaccines in this effect?
      • Was there any difference in the effectiveness of the vaccines between the Alpha and Delta variants?
  • This is what they found: 
  • To explain this graph, look at the orange bars (Pfizer vaccine)
    • After two doses of the vaccine, compared to people who had not been vaccinated, the rate of having a symptomatic infection was 
      • 0.06 (or if you like 6%) for the Alpha variant
      • 0.12 (12%) for the Delta variant
  • The figures for the AstraZeneca vaccine were
    • 0.26 (26%) for the Alpha variant
    • 0.33 (33%) for the Delta variant
  • The data from Scotland were similar – for the Delta variant
    • Rates of infection after the Pfizer vaccine rates were 17% of those in unvaccinated people
    •  Rates of infection after the AstraZeneca vaccine rates were 39% of those in unvaccinated people 
  • Actually, these are not that different from the vaccines’ success against the original variant: the Pfizer vaccine has a substantial effect on reducing rate of having an infection the AstraZeneca has a lower effect
  • But this data is less important to me than the risk of becoming seriously ill- next question

5) If I do get infected, will my vaccine stop me getting seriously ill with the new variants?

  • The data came from linking everyone who tested positive and self-reported symptoms with the national hospital record systems 
  • The question then is how far did the vaccines protect against having such a serious infection that they needed to be admitted to hospital
  • However (and this is good news) their first result was that , despite high numbers of people being infected, the absolute numbers of people admitted to hospital were very low and hence the reported effects of vaccination on this may not be very accurate
  • Below are the data from England
  • Again look at the orange bars (Pfizer vaccine)
    • After two doses of the vaccine compared to people who had not been vaccinated the rate of being hospitalised was 
      • 0.88 (or if you like 88%) for the Alpha variant
      • 0.34(34%) for the Delta variant
  • The figures for the AstraZeneca vaccine were
    • 0.53 (53%) for the Alpha variant
    • 0.25 (25%) for the Delta variant
  • These results seem very strange!
    • They suggest that protection against hospitalisation was stronger against the Delta variant than the Alpha variant
    • They suggest that protection against hospitalisation was stronger following the AstraZeneca than the Pfizer vaccine
    • But remember these are based on small numbers 

Combining the answers to questions (4) and (5) together

  • My main question is: will my vaccine stop me getting infected so badly, I need to be admitted to hospital
  • The level of protection will thus be based on how much less likely is it that:
    • I will get any symptomatic infection – and –
    • If I have a symptomatic infection, I will need to be hospitalised
  • The calculation is quite complex and I have shown the details in the box below
  • In summary, these data show that the Pfizer vaccine is 95% effective and the AstraZeneca vaccine is 92% effective against the Delta variant

And the take home message:

  • Apologies, it is a complex question that defies a simple answer!
  • The main messages are
    • Both Pfizer and AstraZeneca (and probably Moderna) vaccines are as effective against the Delta variant as they were against other variants
    • But high rates of infection with the Delta variant, because of its greater infectivity, will mean I am a little less protected than I was when other variants predominate 

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2 replies on “Double vaccinated with Pfizer or AstraZeneca: but how protected am I against new variants?”

I’m wondering whether the 92% effectiveness of the AZ vaccine only applies whilst we are compliant with the current restrictions (masking in shops etc, social distancing, hand washing, ventilation, ‘rule of 6’ indoors etc.).

By how much will that effectiveness plummet come 19th July when (probably) no more masks or social distancing and unlimited closer contact all round (indoors included, with ventilation on the way down from the Autumn)?


This is the confusing thing in these data. The vaccines will have their 90% effectiveness but that is in relation to not being vaccinated. With the relaxation of rules, there may very well be many more cases, but that will apply to both vaccinated and unvaccinated people. That is why in my piece the effectiveness of the vaccine is only one part of the risk calculation!

Liked by 1 person

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