COVID cases up but milder infections in vaccinated and in the young

May 13, 2021

This week the ZOE COVID Study figures reflect an updated methodology, accounting for the large number of vaccinated individuals within the data set. 

ZOE now calculates infection rates in both the vaccinated and unvaccinated contributors. The percentage of vaccinated ZOE app contributors that takes part in the infection survey now exceeds 89%, far higher than the rate of vaccination in the general UK population. To calculate incidence rates, ZOE is now adjusting its figures to more closely match the rate of vaccination in the general population to give the new incidence and prevalence estimates.

According to updated[*] ZOE COVID Study figures, there are currently 2,782 new symptomatic cases of COVID in the UK on average, based on swab test data from up to five days ago [**]. This compares to 1,685 daily cases a week ago, an increase of 65% from last week. The figures from last week have also been updated to reflect the new methodology. Please see below for the full regional table.

There are currently 78,263 unvaccinated contributors in the ZOE COVID Study infection survey vs. 599,414 vaccinated contributors.

Tim Spector OBE, principal investigator on the ZOE COVID Study app, comments on the changes to methodology and increase in figures: 

“At ZOE, we pride ourselves on being ahead of the curve and on the role we’ve played in the UK’s response to the pandemic. In light of the huge success of the vaccine roll out it was essential that we addressed our methodology. We have a disproportionately high number of vaccinated individuals among our contributors so we’ve adjusted our figures to better reflect the rate of vaccination in the UK population. 
As a result, the incidence numbers have been revised to be higher and we have also seen a notable 65% increase in figures from last week. While the increase in cases this week is likely to be real, the overall numbers are still just a fraction of what they were at the height of the pandemic. Increases of this size are to be expected with changes in social behaviour as restriction ease. We are also seeing milder forms of COVID-19 within vaccinated groups and younger people presenting with much less respiratory problems than previously.” 

Read on for findings on milder COVID-19.

ZOE DATA SEEING MILDER COVID IN VACCINATED POPULATION 

According to the latest analysis by the ZOE COVID Study team symptoms reported by vaccinated and unvaccinated contributors who entered positive PCR test results, their vaccination status significantly alters the experience of COVID-19 infection. The data found that unvaccinated people were much more likely to experience at least one of the “classic three” COVID-19 symptoms (persistent cough, fever, and loss of sense of smell). 

Within a window of three days prior and 14 days following a positive PCR test:

  • The majority (52.6%) of contributors who have had a vaccine do not experience “classic” symptoms. 50% of people with one vaccine dose and 76% of fully vaccinated contributors don’t report classic symptoms
  • This compares to 30.4% of unvaccinated contributors not reporting classic symptoms.

Within a week of a positive PCR test, unvaccinated contributors also experienced a higher number of symptoms. ZOE and King’s College London have previously presented data suggesting five or more distinct symptoms within the first week of infection are related to increased risk of hospitalisation. ZOE observed:

  • Fully vaccinated contributors reported around 0-1 distinct symptoms within the first week of infection
  • Partially vaccinated contributors reported around 2-4 distinct symptoms within the first week of infection
  • Unvaccinated contributors reported a median of 5 distinct symptoms within the first week of infection

Infected vaccinated contributors were defined as follows:

  • People who received first vaccine dose at least 12 days before testing positive for COVID-19 (to give sufficient time for immunity to build, based on trial data)
  • People who received their second vaccine dose at least seven days before testing positive (to give sufficient time for immunity to build, based on trial data).

The figures given above are raw numbers for ZOE COVID Study contributors and have not been adjusted or extrapolated to a wider population.

Tim Spector OBE, lead scientist on the ZOE COVID Study app and Professor of Genetic Epidemiology at King’s College London, comments on the latest data:

“Given the reported rise in daily new cases this week, it is reassuring that we’re seeing much milder infections than those seen prior to the vaccine rollout. The majority of vaccinated people, when infected, don’t experience classic respiratory problems and experience fewer symptoms overall, which means rising infections have not yet resulted in an increase in hospitalisations or deaths and pressure on the NHS. This is a signal that the game has changed, with vaccines allowing us to live much more normal lives in a world with COVID-19.”  

The app is delivered in collaboration with King’s Health Partners, an Academic Health Sciences Centre based in South East London.

The ZOE COVID Symptom Study UK Infection Survey results over time - updated methodology.

Incidence (daily new symptomatic cases)[*], R values and prevalence regional breakdown table - updated methodology 


The ZOE COVID Study incidence figures (new symptomatic cases) are based on around one million weekly reporters and the proportion of newly symptomatic users who have received positive swab tests. The latest survey figures were based on data from 5,221 recent swab tests done between 24 April to 08 May 2021. The data excludes the lateral flow tests.

Map of UK prevalence figures - updated methodology 


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