At first glance, that looks concerning. But a second analysis told a different story. Comparing children who received two full doses of the vaccine with unvaccinated children, the researchers found a hazard ratio of 1.17 — meaning vaccinated children appeared to have a 17% higher rate of diabetes — but this result was not statistically significant. In plain terms, it could easily have arisen by chance.

The negative-control test

To probe the apparent increase further, the researchers turned to a clever check. They examined diabetes incidence around two other changes in Norway's vaccination program that should have no connection to type 1 diabetes: the introduction of the hepatitis B vaccine in 2016 and the switch from a 7-valent to a 13-valent pneumococcal vaccine in 2011. These served as negative controls.

If the rotavirus vaccine truly increased diabetes risk, the pattern should appear only after its introduction. Instead, the researchers saw similarly sized increases in type 1 diabetes following both unrelated vaccine changes.

The authors' conclusion: the 11% rise seen in children born from September 2014 to December 2019 probably reflects broader environmental shifts rather than the vaccine. "We do not think any of the changes in the national vaccination program or the combination of changes explain the increase in type 1 diabetes," they write.

Ruling out other suspects

The team also considered whether the COVID-19 pandemic could account for the rise, since Norway — like many countries — saw a temporary increase in type 1 diabetes incidence in 2022. However, a recent study of Swedish and Norwegian adolescents found no increase in diabetes incidence after COVID-19 infection.

Another candidate was a change in Norwegian prenatal care guidelines in June 2018, when doctors began measuring serum ferritin — a more sensitive indicator of iron status — in the first trimester of pregnancy. High iron levels are suspected to be linked to type 1 diabetes. But the researchers note that iron levels are usually low in pregnancy, and only children born in 2019 would have been affected by the new guidelines, making this an unlikely explanation.

What it means

Some earlier studies had suggested rotavirus vaccination might actually protect against type 1 diabetes, while others found no effect — the evidence has been conflicting. This study, one of the largest of its kind, adds weight to the view that there is no meaningful association in either direction.

"We do not believe that rotavirus vaccination increases the risk of type 1 diabetes," the authors state. They call for additional studies in different countries using different vaccine types before firm conclusions can be drawn, and they emphasize the need for research into other environmental factors that could be driving the long-term rise in the disease.

For parents, the practical takeaway is reassuring: rotavirus vaccines remain a proven tool for preventing severe diarrheal illness, hospitalizations, and deaths in young children, and this study offers no credible evidence that the shots raise diabetes risk.