New wave despite vaccine success: how likely/how large?

Vaccine roll out success, coupled with adherence to behavioural rules, has led to substantial declines of infections in the UK and Israel.  Increasing vaccine coverage in North America and Europe is seen also as the route back to normality.  Yet infectious disease epidemiologists are still worried about a third wave causing large numbers of severe cases.  Are they right to be worried?

Let’s start with the current success

  • Excluding countries in the Far East and Australasia, globally most other countries have had substantial second peaks of infection
  • I have plotted below the rates/million total population (including children) of new diagnosed infections in Israel and UK 
  • The UK – with one of the most stringent and (probably) adhered to lockdown measures since the start of January – has seen a substantial fall in the rate of new cases.  Given that only around 10% of the population has had a second jab, then the decline in new cases is predominantly explained by the measures enforced
  • Israel has a second jab rate of around 60% and epidemiological studies, examining the vaccination records of those who have become ill, by contrast, do point to vaccine roll out being the major explanation for the decline
  • Indeed Israel began easing its lockdown rules in mid-February without seeing, so far, any rise in the rate of infection 

So what’s the worry?

  • Worth repeating (and I have!), though obvious – if we all stayed at home then the infection would die out.  So whether the rates of new infection are going to remain controlled as lockdown is released depends on a number of factors
  • I have illustrated these in the diagram below and will consider them all in turn
  • The general point is that we need to consider a range of factors in order to achieve the elimination of the threat of Covid-19 

Vaccine effectiveness

  • In terms of transmission we want the vaccines to stop people being able to pass on the infection those around them
  • We know that the vaccines stop people getting ill, but it is still uncertain by how much the vaccines stop people transmitting the infection
  • Here are some estimates from two epidemiology groups from the UK: Imperial College London and Warwick University
  • ‘All infections’ includes those with and without symptoms 
  • The rates of infection are following the second jab and assume no waning in immunity over time
  • The assumption about waning immunity is probably reasonable over a short time frame, i.e. protection at least until the end of this year
  • These figures are from the rolled out vaccination programmes rather than the earlier clinical trials and are estimates of the effects across all adult age groups 
  • Pfizer probably stops 90% of transmissions and AstraZeneca around 60%
  • A 60% reduction in transmission though is really good if the population rates of infection are low.  Conversely if the population rates of infection are high then, even with a 90% reduction, there is still scope for high numbers of new infections to be passed from vaccinated people

Vaccine coverage

  • Thus far in Israel, UK and USA, the vaccine coverage has been higher than the pessimists feared
  • Concerns about hazards and the roll out to younger populations less at-risk from severe infection might see a decline in take up, although restrictions on travel etc for those without a vaccine might encourage greater take up
  • Based on different sources of data here are the projections (OK they are guesses!) of vaccine take up in the UK:
  • Although these figures seem ‘very good’, as with the effectiveness data above, in periods of high infection even 10% of people who are not vaccinated could contribute to a large number of people being susceptible to infection

Speed of vaccine roll out

  • Again obvious, but the longer it takes to vaccinate the population the greater the number of people who can transmit the infection
  • This is because as mentioned above  vaccinated people, even with a pessimistic (AstraZeneca) protection, are 60% less likely to transmit the infection
  • An interesting ethical issue is that younger people have more social contacts and currently in many countries contribute proportionately more cases of infection than older people
  • Thus it could be argued that to prevent transmission alone, it might make some sense to focus vaccine efforts on those who are younger 
  • Anecdotally this may be what is happening  in some countries in Eastern Europe


  • As is well known, influenza pandemics tend to occur in winter
  • Whether this is because in the summer there is reduced viral survival with the greater  sunlight and/or humidity, or we spend less time outdoors, is unknown
  • The concern from some epidemiologists is that the decline observed now in countries such as the UK may represent a seasonal decline and that come winter there will be a bounce back 
  • This is the effect of season from the Warwick and Imperial guesses
  • These are not large differences but could tip the balance
  • This underlines the importance of achieving high vaccine coverage before the autumn

Impact of new variants

  • It is now well known that the current major variant in UK, USA, Israel and indeed the rest of Europe is the so-called English variant
  • This variant does spread more easily
  • The South African variant also spreads even more easily, but it seems that high rates of the English variant seem to reduce the South African (or indeed the Brazilian) variants from taking a major hold
  • The epidemiologists have in their scenario planning assumed that no new nastier variant will take hold – this is probably reasonable but cannot be relied on absolutely

Social Behaviours 

  • Clearly a return to normal social interchange increases the chances of transmission
  • It was believed at the start of the pandemic that the natural R value, i.e. the average number of cases caused by one person passing it on to others, is around 2.5
  • Countries with their various lockdowns are achieving rates of say between 0.8 and 1.5
  • R values have to be considered against how many cases there are of course, but the worry is that releasing the brakes given all the other issues mentioned above could lead to a greater number of new transmissions.

Putting it altogether

  • Complicated isn’t it!!
  • This is just one model from Warwick attempting to predict the number of cases hospitalised in a third wave this summer: 
  • To be honest they’ve brought so much together in that graph that I’m not going to even try and explain it all ! But there is an important message here
  • The red line has allowed for all the things I have mentioned above (they call it the ‘central assumption’)
  • An increase in R because of any of the issues mentioned would have major effects on the numbers of severe cases (in this graph shown as the how many beds in hospitals in the UK are occupied by Covid-19 patients) 
  • We can’t easily change the effectiveness of the vaccines (they are pretty good), the seasonal impact or the possibility of new variants
  • By contrast, we can ensure maximum and speedy vaccine coverage
  • More importantly, it will be necessary to keep tabs on all the above, to ensure that any return to ‘normality’ is carefully monitored

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6 replies on “New wave despite vaccine success: how likely/how large?”

Can I ask why Chile isn’t in this study? It has a very high vaccination rate, but also a worryingly high continuing Covid transmission and death rate. Clearly there are other factors going on.


Not exactly sure what you are referring to in “this study” and apologies for any confusion. The data from Imperial and Warwick are all modelled data and although they have used assumptions based on the UK experience, given the different parameters mentioned, they could all be plugged in to estimate the outcome for any population


Chile is better vaccinated per capita than the UK. However, it has surging cases of Covid and rising deaths. I know it has worried the UK government. However, there seems to be 2 factors different to the UK. Firstly they used Sinovac, whose efficacy is now being questioned. Secondly, they appeared to have released the lockdown very quickly. The latter argument would seem to reinforce your article.
I just thought that Chile would have made a very good comparison against the UK and Israel.


Sinovac seems to be an issue as do role of possible other variants – they don’t have the widespread molecular testing. Vaccine roll out of second jabs still not that high, or wasnt’ so hopefully may still yet fall


Another very interesting post, thank you Alan. If the natural R value is 2.5, that means that 60% of the population need to be immune for the prevalence to decline (R-1/R), in the absence of behaviour change strategies. The AZ/Oxford vaccine only prevents 60% of transmission. That means that if the only means by which transmission was being prevented was the AZ/Oxford vaccine, 100% of the population would need to be vaccinated, which is unlikely. It’s clear people are also acquiring immunity from the Pfizer vaccine and prior infection, but is this bad news not only for the AZ/vaccine but also for hopes of herd immunity coming to the rescue?


Certainly the maths* suggests that we need an immunity of 60% to achieve herd immunity. To be honest we don’t really have robust data on how much prior infection prevents someone acquiring and transmitting a second infection. It is interesting that no-one is now talking about herd immunity as a target and that the vaccines’ goal is to stop people getting sick. That is reasonable from a public health perspective. .
* I think the formula is (1-1/R)


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