Public Health Scotland vaccine surveillance data
Letter from Deputy First Minister and Cabinet for Secretary COVID Recovery - Public Health Scotland vaccine surveillance data, 16 March 2022
Dear Siobhian,
During my appearance before the COVID-19 Recovery Committee on 24 February 2022, I agreed to write to you with a response to the question from John Mason MSP regarding the publication of vaccine surveillance data by Public Health Scotland (PHS).
Due to the increasing risk of the data being misinterpreted, PHS will no longer report a weekly summary of COVID-19 cases, hospitalisations and deaths by vaccination status.
PHS continues to seek to provide the most accurate information available to best support the response to the pandemic. PHS will continue to provide updates from the latest scientific analyses and reports on the effectiveness of COVID-19 vaccines.
Since July 2021, PHS has reported COVID-19 cases, hospitalisations, and deaths by vaccination status in the weekly COVID-19 statistical report. These data were first published to help monitor the impact of the vaccination programme on the pandemic, impact on the NHS, and to help understand where to prioritise vaccination delivery. While PHS has stated that the data in the report should not be used as a measure of vaccine effectiveness, PHS is aware of inappropriate use and misinterpretation of the data when taken in isolation, without fully understanding the limitations of the data.
Due to the increasing risk of misinterpretation from growing complexities as the COVID-19 pandemic enters its second year, PHS has taken the decision to no longer report COVID-19 cases, hospitalisations and deaths by vaccination status on a weekly basis. PHS is currently reviewing the content and frequency of reporting this information.
Recent changes in testing behaviour and policies, including the fact that asymptomatic people no longer need a COVID-19 PCR test, limit PHS’s ability to robustly identify and monitor COVID-19 cases by vaccination status. This means PHS cannot confidently compare recent case trends to historical data. There are several reasons why it is inappropriate for PHS to continue to publish case rates, which are set out in the PHS weekly report and I have provided them below.
- COVID-19 rates do not account for potential differences between populations such as behaviour towards social distancing, underlying health issues, or approaches to testing and how much they contribute to COVID-19 rates. For example, a study found that people with two doses of vaccine were more likely to test themselves for COVID-19 compared to those unvaccinated or with one dose of COVID-19 vaccine. This means that unvaccinated people may be less likely to test and report as a case resulting in lower infection rates among the unvaccinated. Comparison of case rates is therefore now inappropriate.
- The population of Scotland is relatively small compared to other countries such as England and the United States, and with a high vaccination rate, systematic underlying differences between the unvaccinated, partially vaccinated and booster populations become more evident sooner than bigger countries that may have a larger unvaccinated population. In Scotland most people are vaccinated and have received a booster, this results in a small number of people who are unvaccinated or have had one dose or two doses. Small increases in case numbers in a small population have a bigger impact on case rates than small increases in a larger population (e.g. boosted) where such differences are less noticeable due to the sheer number of people. Comparison of case rates is therefore inappropriate.
- Vaccinations have not been given to the entire population at the same time with certain groups given priority before others. People are on different timelines since vaccination, dose schedules (e.g. some groups are not eligible for a booster) and vaccine types creating difficulties when making broad comparisons across the population by vaccine status. A number of people have not completed or exceeded their recommended dose schedule. This leads to reduced protection from the vaccine over time. Such differences between people impacts how we interpret the rates by vaccine status, for example the level of vaccine protection is expected to be higher in someone who had their first dose four weeks ago compared to another who had their first dose one year ago and didn’t have the required second dose as per the vaccination schedule. Comparison of case rates is therefore inappropriate.
- Individuals who have not completed their vaccine schedule may be more susceptible to a severe outcome and could result in higher COVID-19 case, hospitalisation and death rates in the first and second dose vaccine groups. For example, some of the older individuals who have exceeded the recommended time will have not received their next vaccine dose because of frailty or ill health. They may, therefore, be more likely to be hospitalised or die if they get COVID-19. Comparison of hospitalisation and death rates is therefore inappropriate.
- There is a lot uncertainty about the number of people in the unvaccinated population because we do not know if individuals are still resident in Scotland. This is unlike the vaccinated population who have had recent contact with the NHS due to vaccination. As case rates require accurate population estimates, the uncertainty in the unvaccinated population will result in an underestimate of case rates. Comparison of case rates is therefore inappropriate. This is explained in more detail in this PHS blog post, which also discusses the issue mentioned at the Committee of the number of people registered with a GP exceeding the population.
I hope that this information is helpful. Vaccine effectiveness analysis is key to understanding whether the vaccination is working, and PHS publishes the latest evidence on this in their report each week. Vaccination continues to weaken the link between new cases and hospitalisations and serious illness. Triple dose vaccination continues to provide a high level of protection against infection and hospitalisation from the virus.
Yours Sincerely,
John Swinney
Deputy First Minister and Cabinet Secretary for COVID Recovery,
Scottish Government