A Dynamic Model of Vaccine Compliance: How Fake News Undermined the Danish HPV Vaccine Program

"Increased vaccine hesitancy presents challenges to public health and undermines efforts to eradicate diseases such as measles, rubella, and polio....It is important to understand the underlying causes of vaccine refusal, because these may be prevented, or countered, in a timely manner by educational campaigns."

It has been documented that media can influence important personal decisions, and there is a nascent public health literature associating human papillomavirus (HPV) vaccine hesitancy with media coverage. This article presents a dynamic model of vaccine compliance that can help pinpoint events that disrupt vaccine compliance. The researchers apply the framework to data around the Danish HPV vaccine media drama, which led to a sharp decline in compliance following the broadcast of a controversial TV documentary. The model demonstrates how media coverage can predict vaccine uptake.

The data are weekly HPV vaccine initiations by birth year cohort from January 2009 to June 2017. The structure of the data leads the researchers to develop a model of vaccine uptake that:

1. Characterises baseline vaccine uptake and is identical for all birth year cohorts; and
2. Characterises the variation in vaccine uptake over time, which can be used to monitor vaccine compliance in real time; it is the time variation in this component that the researchers relate to data on media coverage.

The empirical analysis reveals a great deal of time variation in vaccine compliance, including a sudden drop in 2015. In short, the results show that the decline in vaccine compliance began after negative coverage of the HPV vaccine appeared in the Danish media and, by augmenting the model to include media coverage, the researchers show that media coverage significantly predicts declines in vaccine compliance.

Specifically, a detailed review of Danish media coverage of the HPV vaccine, which is available in a web-appendix, reveals that coverage was overwhelmingly positive until April 2013. The first article in mainstream media that associated the HPV vaccine with serious side effects was published in Politiken, a leading Danish newspaper, on April 17 2013. The article featured a story about a girl presenting symptoms such as frequent headaches, dizziness, and tiredness, which her parents claimed were caused by the HPV vaccine. This article was followed by a series of others in the same newspaper that discussed the possibility of a link between the HPV vaccine and serious adverse events (AE).

Some newspaper articles were published in the Danish press that, according to the researchers were blatantly false and misleading and thus, they claim, warrant a fake news designation (in the original meaning of "false news"). However, most media coverage was merely reporting on alleged side effects or on the declining vaccine uptake that had resulted from this concern. Although these news articles individually cannot be said to be false, they may collectively misrepresent the risk of vaccine-induced side effects.

The results show that the first decline in HPV vaccine uptake coincides with the period in which the Danish media began running stories that associated the HPV vaccine with serious side effects, and the largest decline in vaccine uptake occurred immediately after a vaccine-critical TV documentary, De Vaccinerede Piger - Syge og Svigtede (The Vaccinated Girls - Sick and Abandoned), was aired on TV2 Denmark on March 26 2015. After the programme was aired, Danish HPV vaccine compliance fell from over 90% to less than 30%.

The evidence that media coverage influenced vaccine uptake is strengthened further by the empirical results from an augmented specification the researchers carry out that includes a media coverage variable. For this specification, they find that the media variable is a significant predictor of HPV vaccine uptake. The larger the number of media stories that associate the HPV vaccine with side effects, the lower the expected HPV1 vaccine uptake in subsequent weeks. The empirical evidence points to the TV2 documentary as the main culprit with regard to the sharp decline in vaccine uptake in 2015. Per the researchers, this is primarily because the sharp decline in HPV vaccinations occurred immediately after the documentary aired, and because, they assert, there is no good alternative explanation for the decline to have occurred at that particular point in time. It is also consistent with the empirical results from the augmented specification, because a large number (255) of media stories in 2015 referenced the TV2 documentary, including 41 stories produced by TV2 Denmark. In February 2018, TV2 Denmark acknowledged that their documentary contributed to the failure of the HPV vaccination programme, but emphasised that this had not been its intention.

The model of HPV vaccine uptake also enables quantification of the decline in vaccinations, relative to an imagined scenario in which compliance stayed at the level before the documentary. The researchers estimate that nearly half of the girls born in 2003 postponed HPV vaccination following the TV2 documentary, and many of these girls are still unvaccinated. By the end of the sample period, of the unvaccinated girls born in 2013, nearly 14,000 of these can be attributed, per the researchers' estimation, to the decline in HPV vaccine uptake that followed the TV documentary. For illustration, a 70% reduction in cervical cancer for 14,000 Danish females will, on average, translate into about 100 fewer cases of cervical cancer and 26 fewer deaths. The full consequences of the decline in vaccine uptake may be substantially larger, because several other birth year cohorts, including girls born in 2004 and 2005, are also behind in vaccine coverage, relative to older cohorts.

Having established that Danish media coverage of the HPV vaccine and suspected side effects is a significant predictor of declining vaccine uptake in Denmark, the researchers make some international comparisons, noting that the HPV vaccine programmes in the neighbouring countries of Sweden and Norway did not see much variation in uptake. They also compare the Danish media coverage with a similar episode in the United States (US).

The researchers note that, in the case of Danish HPV vaccination, social media likely played some role in the decline in HPV vaccine uptake and, in fact, were a catalyst for much of the media coverage. Links to the TV2 documentary (on YouTube) and articles that related the HPV vaccine to serious side effects were frequently shared on social media. They suggest that an investigation into the role of social media and HPV vaccinations could be the focus of future research.

In conclusion, the researchers argue that the dynamic model for vaccine compliance they have developed, which is driven by discrepancies between expected and actual vaccination rate, could serve as a tool for health authorities to monitor vaccine hesitancy in real time.