Fact Check- Danish study did not conclude that COVID-19 vaccines adversely impact immune systems or that COVID-19 vaccines are completely ineffective against the Omicron variant

A Danish preprint study did not conclude that mRNA COVID-19 vaccines harm the immune system nor did the authors conclude that COVID-19 mRNA vaccines are completely ineffective against the Omicron variant. However, many on social media have misinterpreted vaccine effectiveness (VE) estimates published in one table within the research.

VE is a measure of the extent to which a vaccine prevents infection. If a VE estimate is 90%, it implies that the vaccine prevented 9 out of 10 infections that would otherwise have occurred, according to Dr Christian Hansen, one of the preprint study’s co-authors.

The preprint, viewable , here and here , shows early estimates of VE for the BNT162b2 (Pfizer-BioNTech) and mRNA-1273 (Moderna) vaccines against infection with SARS-CoV-2 Delta and Omicron variants. This is after analyzing data from Danish databases between Nov. 20 and Dec. 12, 2021.

The estimates in the study are based on a comparison of infection rates between vaccinated and unvaccinated people in Denmark during the days after the Omicron variant was first detected in the country.

Users have shared a screenshot of a table on page 6 of the preprint (a study that has yet to be peer-reviewed) with claims that the data is proof that vaccines cause harm to immune systems (here).

The table shows the number of cases recorded of Omicron or Delta with VE estimates listed according to the time since second vaccine dose.

VE against Omicron was estimated at 55.2% (14 cases) and 86.7% against Delta (171 cases) between 1 and 30 days after second Pfizer-BioNTech vaccine while VE against Omicron was 36.7% (4 cases) and 88.2% against Delta (29 cases) following Moderna second dose.

The VE dropped significantly for the last time bracket - those who tested positive for Omicron or Delta variant and received their second COVID-19 vaccine 91 to 150 days previously.

VE against Omicron was -76.5% (2,851 cases) and 53.8% against Delta (34,947 cases) after the second Pfizer-BioNTech vaccine dose with VE against Omicron at -39.3% (393 cases) and 65.0% (3,459 cases) following second Moderna vaccine.

VE increased following a booster vaccine, with VE against Omicron estimated at 54.6% (29 cases) and 81.2% (453 cases) against Delta 1 to 30 days post-booster and VE estimated 82.8% (5 cases) following a Moderna booster.

The study did not analyze data for severity of disease between unvaccinated, double-vaccinated, and boosted groups.

Users online claimed that the table on page 6, particularly negative VE estimates against Omicron 91 to 150 days following second mRNA dose, was proof that vaccines were harming immune systems with other claims that unvaccinated people were less likely to get infected with the virus.

One user said on Twitter: “Observational study from Denmark indicates that the mRNA vaccines protect for a few weeks only but then SIGNIFICANTLY AUGMENT Omicron infectivity. Is the immune system actually harmed, creating shot repetition dependency (with unknown long-term safety)?” (here).

The tweet gathered more than 3,500 likes at the time of writing.

Another user tweeted: “New study from Denmark shows the vaccines have NO effectiveness against Omicron after just 30 days.” (here).

The study did not conclude that mRNA vaccines harm the immune system, nor did the preprint conclude that vaccines are completely ineffective against Omicron or Delta variants.

The aims of the study were to “determine whether there was any evidence of vaccine protection against Omicron infection after a primary vaccination series and booster vaccine” and to “investigate evidence of waning vaccine effectiveness over time,” Dr Hansen, Assistant Professor of medical statistics and epidemiology at the London School of Hygiene and Tropical Medicine (LSHTM) and co-author of the preprint study, told Reuters in an email.

“Our study provides evidence of protection against infection with the Omicron variant after completion of a primary vaccination series with the BNT162b2 or mRNA-1273 vaccines in the first months after primary vaccination. However, the VE is significantly lower than that against Delta infection and declines rapidly over just a few months. The VE is re-established upon revaccination with the BNT162b2 vaccine,” he said.

Potential bias in VE estimates means that “it is measuring something different from what we think it is,” Dr Hansen told Reuters. “The VE estimate may be biased if the infection rates in the vaccinated and unvaccinated populations are impacted by effects other than the vaccines.”

“The fact that the estimated VE is negative during the last period suggests that there is bias in the comparison between the vaccinated and the unvaccinated population. We also make this point in the discussion. Such biases are quite common in VE estimation from observational studies based on population data (unlike a phase 3 randomised trial which is the gold-standard),” Dr Hansen said.

The discussion section of the preprint detailed caveats of the report and indications as to why VE for Omicron 91 to 150 days following second dose were recorded as negative.

“The negative estimates in the final period arguably suggest different behaviour and/or exposure patterns in the vaccinated and unvaccinated cohorts causing underestimation of the VE. This was likely the result of Omicron spreading rapidly initially through single (super-spreading) events causing many infections among young, vaccinated individuals,” the authors wrote in the study.

“Denmark was very quick to conduct sequencing and to identify the first generations of Omicron cases in the country. Cases during this period occurred to an exaggerated extent in those who were travelling internationally, and those in the social and professional circles of travellers, and were largely vaccinated. We expect therefore that there was an overrepresentation of vaccinated people among the first generations of Omicron cases identified in Denmark, not because the vaccines weren’t protective, but because the variant hadn’t spread far enough into the general population, including into the unvaccinated population, to make for comparable infection rates.” Dr Hansen told Reuters.

Meanwhile, “if socialising in larger groups is more common among the vaccinated than the unvaccinated population, then the infection rate will be higher in the vaccinated population because they are much more likely to be exposed to the virus, not because of their vaccination status,” he added.

“VE estimation relies on vaccinated and unvaccinated individuals behaving in a similar fashion in their every-day lives with respect to COVID-19 precautions and exposure to infection risk. It is conceivable that the increasingly small cohort of unvaccinated individuals that remains in Denmark takes further precautions (precisely because they are not well protected), engage less in social activities, etc. Such discrepancies in risk behaviour between vaccinated and unvaccinated individuals will lead to an underestimated VE.”

Dr Hansen told Reuters: “On that basis it is reasonable to expect that the vaccine effectiveness estimates presented in our study are too low, not only for the fourth period (91-150 days after vaccination) but likely also for the earlier time intervals. To conclude, the vaccines’ protective effect may be low against infection with Omicron after 4 months, but it is most unlikely to be negative!”

Reuters reporting of the preprint study can be seen (here).


False. A study conducted using Danish data between Nov. 20 and Dec. 12 did not conclude that mRNA vaccines cause harm to immune systems. The preprint found that VE against Omicron is significantly lower than Delta and declines rapidly a few months after the second COVID-19 vaccine dose, but is restored following a booster. Negative VE estimates in the final period against Omicron suggests bias in comparison between vaccinated and unvaccinated populations, a co-author of the study told Reuters.

This article was produced by the Reuters Fact Check team. Read more about our fact-checking work here .