The COVID-19 pandemic saw a surge in online misinformation, encompassing false claims, conspiracy theories, and promotion of pseudoscientific cures. This infodemic posed a significant challenge to public health efforts, impacting health behaviors and potentially fueling violence and discrimination. While research has analyzed misinformation content and spread, understanding individual responses remains crucial. This study investigates how age groups process and respond to various 'shades of truth' in COVID-19 (mis)information on WhatsApp, a popular messaging app in the UK and Brazil – two countries severely impacted by both the pandemic and the spread of misinformation. Prior research on age and misinformation susceptibility is inconclusive, with some studies suggesting older adults are more vulnerable due to cognitive decline or source monitoring difficulties, while others find equal vulnerability or even greater resilience in older adults due to extensive knowledge bases. The study thus examines the association between age and responses to COVID-19 misinformation, focusing on belief, perceived credibility, and intention-to-share across age groups exposed to different types of misinformation.

Existing research on misinformation and age reveals mixed findings. While studies suggest older adults share more misinformation on platforms like Facebook and Twitter, other research shows comparable vulnerability or even greater resilience in older adults when evaluating headlines or offline scams. The ability to distinguish true and false information appears to increase with age, but repeated exposure to misinformation, common on social media, increases vulnerability, especially in the absence of strong prior knowledge. Another aspect of the literature involves the different forms misinformation takes online. These range from completely false information, contradicted by expert consensus, to partially false or incomplete information, containing a mixture of true, unverified, and false details. The efficacy of corrective information is also debated, with some studies demonstrating backfire effects, while others showcase the success of corrective strategies that include retraction with alternative explanations or factual elaborations. The source of corrective information plays a role, with expert sources (like WHO) proving more effective than social peers.

The study employed two randomized online experiments in the UK and Brazil using a 2 (age: 18-54 years vs. 55+ years) x 3 (misinformation type: full falsity, partial falsity, full truth) x 2 (exposure: initial exposure to misinformation vs. exposure to WHO corrective information) mixed experimental design. Participants (725 UK, 729 Brazil) were recruited through Qualtrics, and quota sampling ensured even distribution across groups. Stimuli resembled WhatsApp forwards, with three versions representing different 'shades of truth' about garlic curing COVID-19. Exposure 1 involved random assignment to one misinformation type, followed by questionnaires assessing belief, credibility, and intention-to-share. Exposure 2 presented a WHO infographic correcting the misinformation, followed by the same questionnaires. COVID-19 knowledge was measured before Exposure 1. General linear models (GLM) with repeated measures analyzed the data, examining main effects and interaction effects of age, misinformation type, and exposure on the outcome variables.

The study involved 1454 participants, with a higher proportion of males in both countries. COVID-19 knowledge was higher in the UK than in Brazil, and significantly higher among older adults in both countries. Regarding misinformation belief, younger adults showed significantly stronger belief in the misinformation (garlic curing COVID-19) than older adults in both the UK and Brazil. In the UK, a significant age x exposure interaction emerged: belief increased among older adults after corrective information exposure. In Brazil, a significant interaction was observed between misinformation type and exposure: belief increased in the full-truth group after corrective information. For perceived credibility, younger adults rated messages as more credible than older adults in both countries. Misinformation type also affected credibility, with the full truth rated highest. Exposure to corrective information significantly increased credibility ratings in both countries. Significant interaction effects between age and exposure and between misinformation type and exposure were observed. Regarding intention-to-share, younger adults were significantly more likely to share messages than older adults in both countries. Misinformation type and exposure to corrective information also significantly affected intention-to-share, with the full truth and corrective information leading to higher sharing intentions. Several significant interaction effects involving age, misinformation type, and exposure were also found.

The findings challenge the assumption that older adults are inherently more vulnerable to misinformation. Younger adults displayed stronger misinformation beliefs and a greater likelihood of sharing, potentially due to less developed critical evaluation skills or lower existing knowledge. The backfire effect observed among older adults in the UK after corrective information exposure warrants further investigation, possibly linked to repeated exposure enhancing claim familiarity. The effectiveness of WHO corrective information in enhancing credibility and sharing intentions highlights the importance of expert sources in countering misinformation. The study also underscores the dangers of partially true misinformation, leading to higher belief and sharing.

This study contributes to our understanding of how age and different types of misinformation affect responses to health-related claims on social media. The findings emphasize the need for age-tailored interventions targeting younger adults and carefully designed corrective messaging for older adults to avoid backfire effects. Future research should explore a wider range of misinformation types and utilize digital platforms for more ecologically valid experiments. Collaboration between researchers and organizations like the WHO is essential to develop and evaluate effective misinformation correction campaigns.

The study's limitations include focusing on a single misinformation claim, employing simulated WhatsApp forwards, and having a non-representative sample due to oversampling of the 55+ age group. The use of online survey methods also limits the ecological validity of the findings.