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Regression analyses should be interpreted in the context of their limitations and not be used to draw causal conclusions. Given that this is precisely what we do in our manuscript, we were surprised by Nandy et al.’s interpretation that “mandatory vaccination led to increased vaccination coverage,” injecting causal assumptions where none are made. The authors also question our methodology and yet only state that they “re-ran” our analyses with no additional detail. Therefore, we cannot comment on the accuracy and robustness of their analysis or provide any direct comparison between their unshared analysis and our own peer-reviewed approach.

Despite the inability to draw causal conclusions from regression analyses, we would argue that observational studies are a key component of exploring and understanding the relationships between policies and health outcomes and should not be excluded from discussions of policy impact. Such analyses are frequently used by organizations like UNICEF to support policy recommendations, as seen in the Lancet supplement published as part of the Global Breastfeeding Initiative.1

Observational studies, in conjunction with all available evidence, should be assessed when making a policy recommendation. We do not claim that our study alone provides definitive evidence of the effectiveness of vaccine mandates nor that it should be interpreted that way. However, vaccine mandates are not always just a “quick political fix,” as Dr. Nandy and colleagues call them. Whatever political considerations UNICEF may have, our role as academic entities is to collect and present evidence and we would cite the existing body of literature on the effectiveness of vaccine mandates in certain contexts, of which our study is one piece.2

However, vaccine mandates are not a panacea, should not be implemented in isolation and are not always the correct approach. We have argued previously that care must be taken when considering the implementation of a vaccine mandate, particularly in contexts where trust in government is low or the resources are not available to make vaccines easily accessible to the population.3 Furthermore, although we did find a positive association between fines and vaccine uptake, we have argued that stakeholders should seriously consider the consequences of penalties for non-compliance.3 Although our approaches were different, our conclusions are in line with those of the Sabin Vaccine Institute (disclosure – Dr. Omer serves on the board of the Sabin Vaccine Institute), which found that legislation can be effective at increasing vaccine coverage but needs to be paired with other key functions.4

Although it is important to identify and address the underlying causes of low vaccine uptake and vaccine hesitancy, as Brewer et al. found in a review of interventions to increase vaccine coverage, interventions such as vaccine requirements that “intervene on behavior directly without trying to change individual’s thoughts and feelings” are remarkably effective in comparison with interventions targeted at changing attitudes alone in high-income countries (p. 185).5 In order to attain our common goal of universal high vaccine coverage, we should consider all options, including vaccine mandates, provided they are implemented judiciously and in settings where they do not exacerbate inequities.

1. Victora CG, Bahl R, Barros AJD, et al. Breastfeeding in the 21st century: epidemiology, mechanisms, and lifelong effect. The Lancet. 2016;387(10017):475-490.
2. Increasing Appropriate Vaccination: Vaccination Requirements for Childcare, School and College Attendance. Community Preventive Services Task Force;2016.
3. Omer SB, Betsch C, Leask J. Mandate vaccination with care. Nature. 2019;571:469-472.
4. Institute SV. Legislative Approaches to Immunization Across the European Region: December 2018. https://www.sabin.org/sites/sabin.org/files/sabin_european_immunization_.... Published 2018. Accessed February 17, 2020.
5. Brewer NT, Chapman GB, Rothman AJ, Leask J, Kempe A. Increasing Vaccination: Putting Psychological Science Into Action. Psychological Science in the Public Interest. 2017;18(3):149-207.

Vaz and colleagues (1) used regression analysis to conclude that mandatory vaccination led to increased vaccination coverage and reduced measles incidence in Europe. They compared 21 countries classified without mandatory vaccination and seven countries classified with: Bulgaria, Czech Republic, Hungary, Latvia, Poland, Slovenia and Slovakia. The authors found positive association between vaccination coverage and size of monetary fines imposed for not taking children for vaccination.

We have several reservations on the methodology and analysis. While the seven countries have indeed reported high coverage rates for several decades, we do not believe that mandatory vaccination is the primary reason. Instead, strong immunization programmes is the major factor, with fines rarely levied in the countries with mandates. In Hungary, for example, an important reason for high coverage is that pediatric nurses make home visits to new parents, keep children on record, and follow up with defaulters (2).

Regression analysis only shows correlations and not causal effects. Rutter warns against papers like the one by Vaz et al: “From an early point in their training, all behavioral scientists are taught that statistically significant correlations do not necessarily mean any kind of causative effect. Nevertheless, the literature is full of studies [containing direct or implied causal conclusions] that are exclusively based on correlational evidence.” (p. 377) (3).

Regression results are only valid if the model holds. Unfortunately, the authors do not show any metrics to validate their confounders. This is particularly concerning given the small sample size of seven countries with mandatory vaccination and a relatively large number of controlled variables. Moreover, Table 2 shows considerable outliers in the non-mandatory group, but no adjustments are made.

Differences in coverage between the two groups seemed only to be significant before 2013. Indeed, when we ran the analysis with the addition of 2017 and 2018 data, the result does seem not hold. Measles coverage decreased to 93% in Bulgaria, Poland and Slovenia in 2018, compared to 96%-98% in earlier years (4).

In addition, we are concerned about the country categorization. The authors classified countries with mandatory vaccination if measles and pertussis vaccines were mandated, but Belgium, Cyprus, France, Greece, Italy and Malta mandate other vaccines, particularly polio (5). As polio vaccine is combined with pertussis in most European countries, pertussis vaccine is effectively also mandated. Hence, those six countries should have been classified as having mandatory vaccination, which would, of course, have led to different results.

We question the authors’ conclusion that mandatory vaccination has the potential to decrease negative impacts of vaccine-preventable diseases. In our opinion, mandatory vaccination is primarily a political intervention put forward as a quick fix without appreciating and seeking to relieve the root causes of low vaccination coverage. Several of the countries with the best measles control in Europe do not have mandatory vaccination (Netherlands, Norway, Sweden) (4,5). They have inclusive vaccination programmes with follow-up systems for parents who fall outside of the normative behavior. We believe that a public health system should be built on trust and quality services. Threats of penalties do not foster trust.

1 Vaz OM, Ellingson MK, Weiss P, et al. Mandatory vaccination in Europe. Pediatrics. January 2020; e20190620.
2 European Observatory on Health Systems and Policies and the World Health Organisation. The organization and delivery of vaccination services in the European Union. London, UK: World Health Organisation; 2018.
3 Rutter M. Proceeding from observed correlation to causal inference: the use of natural experiments, Perspectives on Psychological Science. 2007;2:377-395.
4 UNICEF, Immunization coverage by antigen (country, regional, and global trends), 2019WHO European Region, Routine immunization profile, WHO Regional Office for Europe, 2019
5 Sabin Vaccine Institute, Legislative Landscape Review: Legislative Approaches to Immunization Across the European Region, December 2018

In the paper ‘Mandatory Vaccination in Europe’, Vaz and colleagues conclude that mandatory vaccination and financial penalties are associated with higher vaccination coverage in European countries. They also conclude that there is an association between mandatory vaccination and incidence rate of pertussis and measles. A pivotal premise for this association study is the assumption that data on disease incidence and vaccination coverage are comparable across European surveillance sites. We find that this assumption is not valid, and we argue that the conclusions should be reconsidered.
The WHO data on vaccination coverage are obtained from different sources, such as administrative systems, surveys or registries. Furthermore, data from many sites are adjusted to account for underreporting. In Norway, data on vaccination coverage is based on entry of all administered vaccinations in a national electronic immunization registry. The data are not adjusted for the WHO report, and are likely to slightly underreport the true vaccine uptake. The differences in data on vaccination coverage that are reported to WHO should be sufficiently acknowledged and discussed when compared across sites.
The incidence of pertussis differs greatly in European countries. Different diagnostic practices and diagnostic availability can partly explain these differences in notification rate, and underscore that data on infectious disease incidence are not directly comparable across European sites. The authors briefly discuss that differences in medical attention seeking behavior and in notification of cases to national surveillance systems can lead to underreporting in sites. However, they assume that these differences are evenly distributed in sites with and without mandatory vaccination. This might be true, but it is also possible that differences in health systems and diagnostic practices leading to under-reporting are associated with sites with a mandatory vaccination policy.
Awareness of pertussis among clinicians in Norway is high and diagnostic testing is easily available for all age groups. The overall notification rate for pertussis has been higher than in all other European countries for the past 20 years. In the study, the average notification rate was estimated to be 73.6 cases per 100 000 individuals per year. This rate reflects notification of cases in all age groups. The main objective for the pertussis immunization programme is to prevent severe disease in infants and young children. Immunity conferred by acellular pertussis vaccines is short-lived, and booster doses are needed to maintain protection. Indeed, in Norway, the notification rate of pertussis declined in 7-10 year olds and in adolescents after introduction of school-entry and adolescent booster doses. Furthermore, Norway has not experienced an increase of cases in infants, as has been seen in other European countries in recent years.
We argue that the authors cannot compare vaccination coverage and notification rates of pertussis and measles across European surveillance sites without acknowledging and discussing the strengths and weaknesses in the data. Furthermore, the overall notification rate of pertussis should not be used to evaluate the impact of immunization or the need for mandatory vaccination of infants and toddlers. The conclusions should be reconsidered.

A comparison of vaccination coverage and outcome with respect to mandatory vaccinations in Europe is basically a comparison between East and West Europe, unfortunately still two completely different realities. Moreover, there are differences between North (high coverage, no mandatory) and South-Europe (low coverage, more states with mandatory vaccinations).
Furthermore, some vaccinations are indirectly mandatory as monovalent vaccines are missing. For instance, pertussis vaccination was not mandatory in Italy before 2017, but tetanus, diphtheria, hepatitis B and polio was. Therefore, pertussis is “mandatory” in Italy. The same holds true for France, Malta and Romania. Finally, ECDC vaccine scheduler has only valid data for mandatory vaccinations in the most recent dataset, but not for older versions.