The literature suggests multiple steps that could be taken by policymakers to decrease vaccine hesitancy and convince doubters to get vaccinated after all. One approach for vaccine advocacy suggests “vaccine adoption = access + acceptance” [17]. Looking at access, it is essential to translate the willingness to be vaccinated into actual vaccination decisions. Our study measured the intention to vaccinate; this rate might differ from actual vaccination uptake (vaccination decision) depending on potential constraints, such as the price of the vaccine and the ease of access of vaccination sites. Vaccines should thus be available in a timely manner and an easily accessible way to have as little attrition as possible [12]. In the case of the coronavirus vaccine, access will prove quite challenging since, at the early stages of availability, the demand for this vaccine worldwide will be much greater than the (short-term) production capacities. Currently, about 5 billion doses of vaccine are produced yearly worldwide, of which 30% are seasonal flu vaccines [18]. So even when a vaccine becomes available, access to it will probably be limited in the short run. Therefore, policymakers need to prepare how access can be organised equitably and effectively.
Our results on acceptability suggest that substantial gains could be made among the sizeable proportion of the population (i.e. 18.9%) that is unsure whether they want to get vaccinated. If this group needs to be convinced to be vaccinated to get to herd immunity, clear communication about safety, and potential side effects of the vaccine is especially important. This could help to stimulate the hesitant part of European citizens to get vaccinated after all.
This is especially important since it is unclear whether the group of people who are willing to be vaccinated in itself is large enough to achieve herd immunity. The basic reproduction number \( R_{0} \) shows the transmission potential of diseases [19], i.e. to how many people the infection is expected to be passed on by one infected individual in a fully susceptible population, on average. The herd immunity threshold describes the proportion of the population that needs to be immune, so that the infectious disease is stable (R = 1) and is calculated as [20]:
$$ {\text{Herd immunity threshold}} = 1 - \frac{1}{{R_{0} }}. $$
This means that the higher the basic reproductive number \( R_{0} \) is, the higher the herd immunity threshold becomes. A recent study estimated a COVID-19 \( R_{0} \) of around 3.87 for Europe [21], implying a herd immunity threshold for Europe of 74%. For the US, it was estimated at around 3.45, implying a herd immunity threshold of 71% [22]; while, a recent study argues these values may be lower if there is heterogeneity in the individual susceptibility to the virus [23]. Of course, these estimates are uncertain, but comparing this 71–74% threshold range with our results indicates that the current willingness levels in France, Germany and the Netherlands, in particular, may prove insufficient to reach this threshold.
Our survey highlighted important differences between citizens from European countries in terms of willingness to be vaccinated against COVID-19. The levels do not follow trends that we see in other vaccination rates, e.g. against measles, which are generally higher, but in most countries below the recommended 95% threshold [24].
Understanding which groups in the population are not willing to be vaccinated and why remains vital for the design of policy responses to vaccination hesitancy. One of the avenues to explore could be to emphasise the social benefits of vaccination more strongly so that they weigh the public health dimension more heavily in their decision whether to vaccinate [13]. A recent study, for example, found that people are more willing to get vaccinated when they were informed that this would protect others who are willing but unable to get vaccinated themselves [25]. Consequently, one of the communication strategies could be to emphasise how vaccination against COVID-19 helps to protect vulnerable members of society. Furthermore, the distribution of vaccinated individuals in the population matters. Pockets of non-vaccinated groups could be highly problematic even when overall vaccination rates are high. Unvaccinated individuals may be more often in contact with other unvaccinated individuals than with vaccinated ones [26]. Examples of measles outbreaks in the Netherlands [27] and the US [28], for instance, show that outbreaks in particular communities may even occur if overall vaccination rates are high, and highlight the role of religious communities and travellers in this context.
Alternative strategies range from restrictive measures against those who chose not to be vaccinated to mandatory vaccination schemes for certain target groups or the whole population. Experimental evidence suggests that individuals under specific conditions may be willing to support mandatory vaccination policies, but this support seems very sensitive to adverse events [29]. Such a policy may be less appropriate in the context of COVID-19.
Beyond finding a vaccine
Our findings highlight that considerable policy effort may be required to come from having a vaccine to adequate vaccination rates, especially in some countries. Targeting those in the population who are currently hesitant seems most promising and cost-effective, but this requires convincing evidence and clear communication on the safety and effectiveness of the vaccine. This may be at odds with the current push for having a vaccine available as soon as possible. A campaign emphasising the social benefits of vaccination could increase the willingness to be vaccinated among those amenable to such pro-social motives. Finally, a sizeable proportion of the population indicates not to be open to vaccination. This group may remain at risk of spreading the virus and contracting the disease, even after herd immunity has been achieved. Concluding, improving our understanding of vaccination hesitancy in the context of COVID-19, as well as finding and using policies to overcome it, may be as important as discovering a safe and effective vaccine.