Even small changes in official health communication can have major consequences for both trust and behaviour regarding vaccination. This is shown by an international study using US data.
When health authorities communicate about vaccines, most people expect the messages to convey sound science and clarity. However, a new international study involving researchers from the University of Copenhagen shows that even subtle changes in language can have significant consequences for how the public perceives both vaccines and the authorities.
The study is based on a change made by the US health authorities, the CDC, in 2025, when communication about vaccines and autism shifted from reflecting scientific consensus to highlighting uncertainty. This led researchers to ask: What actually happens when uncertainty is communicated in an area where the research is fairly clear-cut?
Vaccines and autism
In an online experiment involving nearly 3,000 American participants, the subjects were presented with different versions of the CDC's information on vaccines. Some read the earlier version, which clearly stated that there is no link between vaccines and autism. Others read the newer version, which emphasised that a link could not be ruled out. A third group served as a control group.
The results were clear: participants exposed to the uncertainty-based communication assessed vaccines as more risky and expressed a lower willingness to be vaccinated themselves.
'Our results show that the way one communicates can in itself become a risk factor,' says co-author Lau Lilleholt Harpviken, assistant professor at SODAS and the Department of Psychology at the University of Copenhagen.
'When you highlight uncertainty in an area where there is actually broad scientific consensus, it can create doubt that is out of proportion to the evidence,' he explains.
Consequences extend beyond vaccines
The study also shows that the effects do not stop at the immediate decision regarding vaccination. Participants who read the text emphasising uncertainty generally had lower trust in the health authority behind the information and were more likely to accept classic forms of science denial - such as conspiracy theories or the selective use of research.
'Even a moderate but sustained decline in willingness to be vaccinated can have serious consequences for public health. But it is also worrying that trust in authorities and science may be undermined more generally,' says Lau Lilleholt Harpviken.
The researchers emphasise that the study is not an argument against openness about scientific uncertainty as such. The point is rather that uncertainty should be communicated proportionately and always placed in relation to the total body of knowledge in the field.
'Health authorities simply need to be aware that changes in communication can have major societal consequences. We therefore recommend that new messages be openly documented, carefully aligned with existing evidence and - where possible - tested before being rolled out to the public,' concludes Lau Lilleholt Harpviken.
The study, entitled 'CDC communication undermines trust in vaccines', has been published in the journal Science. It can be read here.
