Comparison of the SARS-CoV-2 infection rate of vaccinated and unvaccinated populations does not reflect the actual effectiveness of the COVID-19 vaccine


SARS-CoV-2 infection rate is higher among the vaccinated than the unvaccinated in the UK


lack of context: Comparison of the COVID-19 infection rate between vaccinated and unvaccinated populations did not take into account potential differences between those groups, such as social interactions or health-seeking behavior. Such differences change the risk of people becoming infected and therefore need to be taken into account to avoid biasing the analysis.


Vaccinated and unvaccinated populations may differ in many characteristics, such as age, population size, social behavior, or health-seeking behavior. These differences should be taken into account when comparing SARS-CoV-2 infection rates between vaccinated and unvaccinated populations. Failure to do so can lead to biased conclusions. Studies accounting for these differences showed that COVID-19 vaccines effectively reduce the risk of getting sick.

COMPLETE CLAIM: The SARS-CoV-2 infection rate is higher among vaccinated than unvaccinated people in the UK; vaccines do not protect against disease


The UK was the first country to authorize COVID-19 vaccines and start the vaccine rollout in December 2020. Since then, 86% of the UK population over the age of 12 has He received at least two doses of vaccine, starting on April 7, 2022.

Although clinical trials and real-world studies found COVID-19 to be effective in preventing illness and death, the impact of the nation’s massive vaccination drive has been a recurring theme of misinformation. Some used a biased comparison of death rates between vaccinated and unvaccinated people to promote inaccurate claims that COVID-19 vaccines weaken the immune system or are not effective in preventing death from COVID-19. Health Feedback has reviewed these claims on several occasions.

Yet claims like this keep coming up. On April 6, US State Senator from New Mexico Gregg Schmedes published a Facebook post stating that “the unvaccinated have the lowest infection rates in the UK”, showing a graph of infection rates in the UK between March 6 and 27, 2022.

As we will explain below, although this comparison used official data, it does not make scientific sense, as it did not take into account other important factors that differentiate vaccinated from unvaccinated people, in addition to vaccination status.

When comparing the rate of cases or deaths between two groups of people, for example between vaccinated and unvaccinated people, it is important to take into account any other differences between them besides vaccination status that could also affect your risk of getting sick. Failure to take these differences into account could lead to a biased result and an incorrect conclusion. Health Feedback previously explained the pitfalls of such comparisons and the steps that epidemiological or clinical studies take to avoid them.

The claim under review is based on the weekly COVID-19 report from the United Kingdom Health Security Agency (UKHSA) on March 31, 2022. It used data from Table 14 of that report, which presented the rate of newly reported COVID-19 cases between the vaccinated and unvaccinated groups between the March 6 and 27, 2022.

However, the UKHSA clearly warned in its weekly report that a crude comparison without further adjustment of the figures cannot provide meaningful information on the effectiveness of the vaccine:

Vaccination status of cases, hospitalized patients, and deaths should not be used to assess vaccine efficacy due to differences in risk, behavior, and testing in vaccinated and unvaccinated populations.

In a blog post, UKHSA list some of the factors that can bias the results of a crude comparison of infection rates. One of them is the fact that the vaccination campaign began by prioritizing people with the highest risk of exposure to COVID-19, such as health workers. This means that people who are more likely to develop COVID-19 are overrepresented among the vaccinated compared to the unvaccinated. This could lead to an increase in the number of cases among vaccinated people that is not related to the effectiveness of the vaccine.

Another factor to consider is the possible difference in behavior between vaccinated and unvaccinated individuals. The UKHSA blog post cited two possible behavioral differences. First, vaccinated and unvaccinated individuals may behave differently with respect to social interactions. For example, vaccinated people may become less compliant with physical distancing measures, which may increase the risk of infection, knowing that they benefit from vaccine-induced immunity. The increased risk of infection in this case is not related to the effectiveness of the vaccine.[1-3].

Second, vaccinated and unvaccinated people may differ in their health-seeking behavior. If a group tends to seek medical attention and get tested more often, the probability of detecting COVID-19 in that group will be higher. Therefore, they may be overrepresented among new COVID-19 cases. This is not because that group is more vulnerable to infection, but because the group gets tested more often.

A preprint, a scientific study that has not yet been peer-reviewed by other scientists, reported that vaccinated people were more likely to seek tests than unvaccinated people.[4]. Therefore, such a difference in behavior may affect the number of new cases of COVID-19 detected in vaccinated and unvaccinated people.

Taken together, these factors indicate that infection rates cannot be directly compared without additional steps to limit risk of bias. In particular, differences in health care-seeking behavior can be explained by using a specific study design called a test-negative study.[5,6].

In fact, the UKHSA fixed that this method was used to assess the efficacy of COVID-19 vaccines taking into account several biases, such as those described in this review:

All of these factors are taken into account in our published analyzes of vaccine efficacy, which use the test-negative case-control approach. This is a recommended method for evaluating vaccine effectiveness that compares the vaccination status of people who test positive for COVID-19, with those who test negative.

This method helps control for different propensity to be tested and we can exclude those known to have been previously infected with COVID-19. We also control for important factors including geography, time period, ethnicity, clinical risk group, living in a nursing home, and being a health or social care worker.”

UKHSA list in Table 5 of their report, several vaccine effectiveness studies and preprints using the test-negative design. They reported that COVID-19 vaccines were effective in preventing disease, with variable effectiveness depending on SARS-CoV-2 variant, booster dose use, and time since vaccination.[7-11].

These analyzes showed that, when taking into account potential factors that can generate biases, the vaccines proved to be effective in protecting against COVID-19, with differences depending on the variant of SARS-CoV-2 and the time elapsed since vaccination. Interestingly, Schmedes’ Facebook post does not mention this analysis.

In summary, a crude comparison of the infection rate between vaccinated and unvaccinated populations leads to misleading conclusions. Although the data presented in Table 14 of the UKHSA weekly report took into account certain factors, such as age and the difference in population size between the vaccinated and the unvaccinated, it did not consider other potential biases that could be produced. due to differences in social conditions. interaction or health-seeking behaviors. More carefully designed studies of the effectiveness of vaccines, which take into account differences between the vaccinated and unvaccinated, as well as vaccination status, have shown that vaccines are effective in preventing symptomatic infections.


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