In Europe, non-European immigrants display lower vaccination take-up than natives, a higher prevalence of STDs, and lower usage of publicly-funded treatments. Therefore, migrants (and especially women) face worse preventable health issues. Qualitative research suggests that migrants' lower take-up might be due to culturally founded concerns, which are then exploited and targeted by anti-vax information. These are adverse effects on fertility and, in the case of the HPV vaccine, which is administered at a very young age to prevent a sexually-transmissible infection, on reputational concerns related to incentivizing pre-marital sexual activity. Existing literature also investigates parental education as an alternative explanation to vaccines' mistrust, finding mixed evidence which suggests a non-linear relationship between education level and willingness to vaccinate children. In particular, qualitative research highlights that targeted misinformation addresses emotionally charged contents to lowly educated parents, and pseudo-scientific content to highly educated parents.

In this study, 7500 mothers of children (girls and boys) who are due to be offered the HPV vaccine in fall 2021 within the Swedish national vaccination program are randomly sampled. Mothers are stratified according to immigration status, and are sampled from the entire relevant population through Swedish population registers. There are 5 strata: (i) Mothers were born in Iran, Iraq, Afghanistan, Somalia, Eritrea (irrespective of education) (N=2500); (ii) Mothers born in Sweden and have attended at most 3 years of high school (N=1600); (iii) Mothers born in Sweden and have attended more than 3 years of high school and have not attended any university (N=1400); (iv) Mothers born in Sweden and have achieved at most undergraduate education (N=1000); (v) Mothers born in Sweden with graduate education after obtaining an undergraduate degree (N=1000). Sample sizes are chosen with the expectation of having 500 actual participants per stratum, based on participation rate estimates from the implementer (Statistics Sweden, SCB).

Within each stratum, the investigators randomize 3 types of information sheets on the HPV vaccine: emotionally-framed (T1), scientifically and statistically framed (T2) and uninformative placebo (C). Along the information sheet, all mothers receive a first survey covering their beliefs on vaccines (henceforth defined "baseline survey" for simplicity, despite being administered along the treatment), which does not vary across treatment arms. The information sheet and the baseline survey invitation are sent by ordinary post in June 2021 to the mother's address, and the survey can be compiled both on paper and online. In July and August, mothers who have still not compiled the baseline survey will receive 3 reminders: each reminders contains the information sheet, which depending on the specific reminder can be printed or accessible in digital format when accessing the online survey. Between September and October, all mothers will have to decide whether to authorize HPV vaccination for their child, which takes places directly at school and free of charge. In November, mothers who have responded to the baseline survey receive an invitation to an endline survey. All information sheets and surveys are available in Swedish, English, Arabic and Farsi. Informed consent is obtained in the baseline survey. For mothers who choose to participate, investigators will obtain, from population registers, background variables concerning the mother herself, the other parent and the child who is due to immunization. These cover specific information on parents' education, income, wealth and occupation, marital status, number of total children, gender of the child due to HPV vaccination, and if she/he received the previous vaccine in the national program's schedule (3 to 4 years before our intervention), order of the child among siblings.