The Hidden Vaccine Gap: Why Willingness Is No Longer Enough

For years, vaccination campaigns have measured public willingness as a sign of progress, but willingness can be a fragile signal. A person may support vaccination in principle and still miss the shot; another may begin from refusal and later decide to vaccinate. After the COVID-19 pandemic, many countries have expanded vaccination infrastructure, improved delivery channels and built seasonal campaigns around high-risk groups. Still, availability alone does not guarantee protection if people delay, reconsider or disengage before receiving the vaccine.

A new study from Portugal suggests that assumption is becoming increasingly unreliable. Among older adults followed through a seasonal vaccination campaign, many who intended to receive COVID-19 or influenza vaccines never did, while some who initially refused vaccination later changed their minds and got the shot.

Published in Vaccines, the study followed adults aged 60 and above in mainland Portugal during the 2023/2024 COVID-19 and influenza vaccination campaign. Researchers surveyed participants before the campaign began and again after it ended, allowing them to compare stated willingness with actual uptake.

According to the study, vaccine hesitancy is not a fixed identity or a single moment of doubt; it shifts across time, differs by vaccine, and responds to trust, perceived risk, convenience, fatigue and advice from health professionals.

The Gap Between Intention and Action

Before Portugal's seasonal campaign began, acceptance appeared relatively strong. Most surveyed older adults said they intended to receive both vaccines, with acceptance at 68.32 percent for COVID-19 vaccination and 72.96 percent for influenza vaccination. On paper, that level of willingness would appear encouraging, especially in an older population that faces higher risks from respiratory infections.

The follow-up survey told a more complicated story. Among those who initially accepted COVID-19 vaccination, 26.14 percent did not receive it. For influenza, 11.48 percent of initial acceptors did not vaccinate. These figures show that willingness can weaken, stall or fail to translate into action even among people who begin a campaign apparently supportive of vaccination.

This is a major operational problem for public health. Campaigns often concentrate their energy on people who are hesitant or openly opposed, while treating those who express acceptance as already persuaded. The Portuguese evidence suggests that this approach can miss a significant group of people who are not anti-vaccine but still do not vaccinate. They may delay because of safety worries, past side effects, doubts about effectiveness, low urgency or practical barriers. They may also reconsider as campaign messages, personal conversations and media narratives evolve.

The study also found movement in the opposite direction, with some people who initially refused vaccination later receiving it. It is equally important because it challenges the assumption that refusal is always final. Even people who begin from a negative position may still respond to trusted advice, changing risk perceptions, reminders or easier access. Vaccination decisions, therefore, should be understood as a process rather than a fixed declaration.

COVID-19 Boosters Are Still Carrying the Weight of Pandemic Fatigue

The study's comparison between COVID-19 and influenza vaccination is especially revealing because both vaccines were offered during the same seasonal campaign and targeted the same older population. Yet hesitant individuals behaved differently depending on which vaccine was in question. Among those hesitant about COVID-19 vaccination at the start of the campaign, 56.50 percent did not get vaccinated. Among those hesitant about influenza vaccination, non-uptake was much lower, at 30.60 percent.

The difference suggests that COVID-19 boosters continue to face a more difficult confidence environment than flu shots. Influenza vaccination is familiar, seasonal and long established. COVID-19 vaccination, by contrast, remains tied to the social and political legacy of the pandemic, including public fatigue, changing booster guidance, safety concerns and a widespread perception that the emergency phase has passed.

For health authorities, the finding argues against treating COVID-19 and influenza vaccination as interchangeable simply because they can be delivered through the same campaign infrastructure. A person may accept the logic of annual flu vaccination while questioning the need for another COVID-19 dose. A campaign that uses the same message for both vaccines may therefore fail to address the specific doubts attached to COVID-19 boosters.

As countries normalize COVID-19 vaccination within seasonal respiratory disease strategies, they will need more precise communication. Flu vaccination may require reminders, convenience and routine risk messaging. COVID-19 vaccination may require stronger trust-building, clearer explanations of booster value and more direct engagement with concerns about side effects, effectiveness and repeated dosing.

Trust, Risk Perception and Fatigue

The study helps explain why people who appear open to vaccination still end up unvaccinated. Among those who initially accepted vaccination but later did not receive it, the main reasons were linked to confidence. Many cited lack of trust in vaccine safety, while others pointed to previous side effects or doubts about effectiveness. This shows that even apparent acceptance can be fragile when people are not continually reassured by credible information and trusted health professionals.

Among hesitant individuals who did not vaccinate, the dominant issue was often low perceived risk. Many did not necessarily reject vaccines outright; they simply did not believe they were personally likely to become seriously ill. In a study focused on adults aged 60 and above, that is a significant concern. Older adults are among the groups most likely to benefit from protection against severe respiratory infections, but campaigns may fail if people do not see the risk as personally relevant.

Vaccine fatigue adds another layer to the challenge, particularly for COVID-19. After years of pandemic communication, repeated doses and shifting public guidance, some people appear less motivated to act even when vaccination remains recommended. This does not mean they are unreachable. It means campaigns need to move beyond generic messages and speak more directly to the reasons people delay, doubt or disengage.

The study also points to practical levers that can still influence behavior. Among those who moved from hesitancy or refusal to uptake, reasons included self-initiative, physician advice, pharmacist advice, risk awareness and SMS invitations. The combination suggests that successful campaigns depend on trusted human relationships as much as logistics. Doctors and pharmacists can address doubts in ways mass messaging cannot, while reminders and accessible vaccination sites can help convert intention into action.

Vaccine Policy Must Become More Adaptive

Although the research is based in Portugal, its implications are relevant to many health systems. Countries are trying to sustain vaccine uptake in an environment shaped by misinformation, pandemic fatigue, weaker institutional trust and growing pressure from aging populations. Seasonal vaccination will remain a recurring public health priority, but the old model of announcing availability and waiting for uptake is no longer enough.

For developing countries and Global South stakeholders, the lesson is particularly important. Expanding vaccine supply remains essential, but access alone will not guarantee coverage. Campaigns also need behavioral intelligence, trusted local messengers, community-based delivery and timely follow-up. In resource-constrained settings, missed opportunities can carry especially high costs because health systems may have fewer chances to reach vulnerable populations during each campaign cycle.

There are also opportunities for innovation and investment. Digital reminders, pharmacy-based vaccination, community outreach, targeted risk communication and data systems that track shifting attitudes can all help close the gap between willingness and uptake. However, technology should support trust rather than substitute for it. An SMS may prompt someone to act, but confidence is often built through a conversation with a clinician, pharmacist or community health worker.

The study has limitations that should temper overgeneralization. Its findings are based on mainland Portugal and adults aged 60 and older. Vaccination status and chronic disease were self-reported, and the subgroup of outright refusers was small, making some estimates less precise. Even with those caveats, the core insight remains highly relevant: vaccine hesitancy is dynamic, and public health campaigns must be designed around that reality.

Nevertheless, the future of vaccination policy will depend on whether health systems can understand people not as fixed categories of acceptors, hesitant individuals and refusers, but as decision-makers moving through uncertainty. If attitudes can shift away from vaccination, public health cannot afford complacency.