Comparison of Response Rates on Invitation Mode of a Web-based Survey on Influenza Vaccine Adverse Events among Healthcare Workers: A Pilot Study

The purpose of this study was to determine whether response to a web-based survey of healthcare workers would be higher with a posted or an emailed invitation. The researchers also report results of the November 2016 pilot study, which aims to estimate the percentage of adults in Toronto, Canada, vaccinated against influenza who report recurrent systemic adverse events (the same systemic adverse event occurring successively following receipt of influenza vaccines).

As is explained here, previous evidence comparing the impact of different invitation modes on response rates of web-based and paper-based surveys is contradictory. Web-based surveys offer advantages over paper-based surveys such as reduced cost, user convenience for automatic skip patterns, and reduced transcription error and data entry costs. However, technical issues can frustrate potential participants if sites are not easily accessible, properly designed, and well maintained. Emailed invitations to web-based surveys have advantages including direct links to survey sites, ease of sending and responding, and lower costs. However, delivery and open rates suffer due to incorrectly recorded or outdated addresses, blockage by spam filters, and lack of perceived legitimacy. Also, since email addresses may not be available for all members of the population of interest, there are concerns about selection bias. In contrast, postal invitations may appear more legitimate and attract greater attention, but are somewhat more burdensome. Postal invitations can also be more costly to the researchers and can suffer from non-delivery due to outdated or incorrect addresses and low response due to perceived importance.

For the study, members of a registry of adults (18 years and older and predominantly healthcare workers) who volunteered to receive information regarding future studies about influenza were randomly assigned to one of eight groups. Four groups received an invitation by email with a direct link to the survey site, while the other four groups received an invitation by post with the URL to the survey site included in the letter. Each invitation arm was randomised to receive one of four versions of the questionnaire: short or long with either open- or close-ended questions. Non-respondents received one reminder using the same mode of contact as their original invitation.

The researchers found that the overall response rate was higher for those sent the invitation by email (34.8%) than by post (25.8%; p< 0.001) and for older versus younger participants (p trend < 0.001). Of those who responded, 387/401 had been vaccinated against influenza at least once since adulthood.

Adverse events following influenza vaccination were categorised into four groups: no adverse event, local adverse event only, systemic adverse event (most recent only), and recurrent systemic adverse event. An adverse event was defined as any untoward, unfavourable, or unintended symptom or disease following influenza immunisation.

Of those responding to the question, 70/386 (18.1%) reported a systemic adverse event after their most recent influenza vaccine, including 22 (5.7%) who reported a recurring systemic event. Systemic adverse events (e.g., muscle aches) were reported more often by males 18-49 years old than by other groups (p= 0.01). More respondents who reported only a local adverse event (93.1%) planned to be vaccinated again next year than those with a systemic adverse event (69.7%; p=0.04).

The fear of adverse events is an often-stated reason for rejecting the annual influenza vaccine. In this study, respondents who experienced a systemic adverse event were significantly less likely to plan to be vaccinated against influenza in the future than those who had only a local reaction. Participants who experienced recurring systemic adverse events were equally likely to plan future influenza vaccinations as people who experienced a systemic adverse event in the most recent season only. This suggests that people may overcome their initial decision regarding revaccination once the chance of an adverse event is weighed against not being protected against influenza the following season.