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Covid-19 infection rates are still high: is there a justification for relaxing mitigation measures?

In the UK, with the pressure on hospital beds from Omicron being containable, the government announced last week a relaxation in the rules brought in to minimise transmission of infection.  In this post I consider why the data might not support this decision

What is happening to infection rates in UK, Europe and North America?

  • There is no shortage of data on reported infection rates in every country
  • In this graph I show the rates of infection since the start of the Omicron wave in 5 selected countries 
  • The broadest conclusion is that over the past 2 weeks, rates have certainly not fallen, and in France indeed are rising
  • Many other European countries are also showing even more substantial rises
  • in total, cases in the past week are 15% up from the previous week across the whole of Europe
  • We need to be careful, though, as these data are of ‘confirmed cases’:
    • If the number of people seeking to be tested falls, for reasons perhaps related to the logistics of obtaining a test – 
    • Then (everything else being equal) these published numbers of confirmed infections may underestimate the true number of cases
    • The opposite may occur, of course, if tests are easier to obtain
  • As an example, though, the UK government on 11th January removed the requirement to have a PCR test if you were asymptomatic despite having a positive lateral flow test
    • Whilst this was not unreasonable given the demand for PCR tests, it inevitably could have resulted in the initial fall in the number of PCR positive cases
    • I have shown, that after January 11 (red arrow on the graph above), there was a fall for a week but that this fall has now levelled off 
    • Given the upward/flat trend in other European countries, I question how accurate are the UK figures, reported daily in the media.
  • Thanks to KS for also reminding me that another inaccuracy in the daily data from England is that the number of cases reported excludes anyone who has been registered as having an infection in the past: reinfection

Can we know the true rates of new infections? 

  • A simple question but a bit complicated to answer (typical epidemiologist!)
  • Fortunately, in the UK, there is the Office of National Statistics (ONS) weekly survey of a random population sample, using PCR tests on everyone in the sample – whether they have symptoms
  • The most recent results, published 3 days ago, provide the best guide to the numbers of people who are positive on the day of testing in the 4 UK countries
  • The graph below shows the data for England from December 1st to January 15th, ie in the period since cases of Omicron started to be seen in the data
  • The data showed that on 15th January, it was estimated that 5% of the population were infected: 1 in 20
  • Remember also a positive PCR does not absolutely indicate a current infection
    • PCR may take a few days to become positive
    • The PCR can then be positive for 2 weeks or longer 
    • Thus ONS in their data assume that across the total number of people who  are PCR positive today, the “average” date they caught their infection is probably about 2 weeks ago
    • Thus, their most accurate assessment of the rate of new infections is on January 1st, which if sustained suggests that each week, at the start of the year, which is consistent with 5% of the population were developing a new infection
  • Now look back at the first graph, which shows the flattening trend over the past couple of weeks
  • Inevitably sophisticated data sets such as the ONS survey are playing ‘catch up’
  • Over the next couple of weeks there may be an important downward trend in new infections, but we cannot be certain yet
  • Conclusion: Difficult to be accurate but even if the weekly rate of new cases is now not quite as high as 5% of the population, it is unlikely to be much lower and the infection must be considered as still unacceptably widespread in the population

Should boosters not have impacted on these trends?

  • An obvious question, but why haven’t the rates dropped since the booster programmes have now been rolled out in many countries?
  • We know that the main effect of the boosters is to reduce severity of the infection
  • Indeed, the report this weekend from the USA showed that triple-vaccinated people had one third of the rate of symptomatic Omicron, (including those with just mild symptoms) compared to doubly vaccinated, which is great news
  • But current vaccines do not have a major effect of preventing transmission
  • The consequence is that being in a room of triple-vaccinated people does not preclude a significant risk of contracting the infection 

Is there a rationale for relaxing measures aimed at reducing transmission?

  • A summary of the above is therefore that there is still lots of Omicron around which can continue to infect fully vaccinated people
  • The data are still being collected on the chance of catching Omicron twice, but it won’t be negligible 
  • For sure, Omicron is milder, especially in fully vaccinated people, and there is a line of argument that says transmission of an infection of that (low) level of severity need not be a public health concern
  • But if we can reduce transmission even of this milder but highly infectious illness, without societal cost, why not?
  • There is a Latin expression which underpins much of medical guidance: primum non nocere (first do no harm)  
  • I mention that because wearing masks on public transport and other places, at a time when there is a background infection rate of 1 in 20, would seem to be a no-brainer
  • For statistically inclined readers, rate of 1 in 20 does not mean that if you are in a gathering of 19 people, you are protected!  In fact, with that background rate, in any gathering of just 10 people, there is a 40% chance that at least one person will be infected

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One reply on “Covid-19 infection rates are still high: is there a justification for relaxing mitigation measures?”

Thanks Alan. Useful to have confirmation that it’s worth wearing my mask. Only one cavill today. “For statistically inclined readers,…”
Statistically inclined readers, one hopes, wouldn’t make such an assumption. Reminds me of my introduction to ‘A’ level stats, which started with “What is wrong with the following statements? If there is a 1 in 100 chance of someone taking a bomb onto the aeroplane and there are 100 places, you should take a bomb on yourself to make sure no-one else does ”
Viv x

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