Study population and recruitment
The survey was conducted from October 15, 2019, to December 31, 2019. The questionnaires for pregnant women were completed during routine antenatal care visits at a mix of 30 public and private clinics or hospitals located in metropolitan areas and the eight provinces of South Korea. Medical doctors or trained assistants distributed either a paper form or an online link to the survey in Google forms in person via opportunistic sampling at antenatal clinics or wards. Questionnaires for physicians were distributed to medical doctors registered with the Korean Society of Maternal–Fetal Medicine (KSMFM), Korean Society of Perinatal Medicine (KSPM), Korean Association of Obstetrics and Gynecology (KAOG), or Korean Society of Ultrasound in Obstetrics and Gynecology (KSUOG) via a paper form or an online link to the survey in Google forms by e-mail. Participation in this survey was voluntary, and no financial or other incentives were offered. Response to the survey implied consent. Information provided by the participants was voluntary and possibly incomplete.
Survey questionnaires
Two questionnaires were developed and used anonymously to survey pregnant or postpartum women and OBGYN doctors (Suppl. 1 & 2). The questionnaires were adapted from previously self-administered questionnaires [16,17,18] composed by a multidisciplinary study team that included OBGYN doctors, biomedical statisticians, and pregnant women. A pilot survey involving both target groups was conducted to ensure questionnaire comprehensiveness. Because some participants did not know that the national free influenza vaccination program in the 2019–2020 flu season included pregnant women, we added the response option “free vaccination program” to question 10–2 in the questionnaire.
The questionnaire for pregnant or postpartum women assessed the following characteristics: age, pregnancy duration, parity, natural conception or use of assisted reproduction, education level, occupation, and administrative district of residential areas. Inclusion criteria were pregnant women over six weeks’ gestation with confirmed fetal heartbeat by ultrasonography and postpartum women within six weeks after delivery. The questionnaire for physicians queried the following characteristics: age, sex, recent maternity care, and characteristics of their employer, including whether their clinic or hospital is private or public and within which administrative district it is located. Residential areas and physicians’ work locations were divided into metropolitan and non-metropolitan areas. Metropolitan areas included Seoul, Busan, Gwangju, Incheon, Ulsan, Daejeon, Sejong, and Kyunggi provinces around Seoul. Non-metropolitan areas included Chuncheon, Gyeongsang, Jeolla, Gangwon, and Jeju provinces.
Pregnant and postpartum women were asked the following: (1) whether they received influenza vaccination during pregnancy in the 2019–2020 flu season; (2) whether they received information about influenza vaccination; (3) information sources; (4) influenza vaccination during a previous pregnancy; (5) reasons for not receiving influenza vaccine; and (6) factors influencing future vaccination. The women were classified according to self-reported influenza vaccination status for the flu season of 2019–2020. Response options for questions (1), (2), and (4) were yes or no. Questions about information sources permitted multiple responses. If a response included OBGYN doctors with or without other sources, it was designated to “OBGYN doctors.” If a response included other sources such as public health, media, friends, or family but not OBGYN doctors, the source was designated to “other sources.”
OBGYN doctors were grouped according to whether they provided routine recommendations for the influenza vaccine for pregnant women based on an affirmative response to the question, “Do you recommend the influenza vaccine to pregnant women in your clinic?”. OBGYN doctors who answered, “always recommend vaccination,” were designated to the, “routine recommendation group,” and OBGYN doctors who responded, “sometimes or never recommended vaccination,” were designated to the, “passive recommendation group.” Physician awareness of the 2019 national free influenza vaccination program for pregnant women and government recommendations such as “all pregnant or breastfeeding women during flu season are primarily recommended to receive an inactive influenza vaccine” was evaluated. Attitudes toward providing information about influenza vaccination for pregnant women were analyzed based on responses to the following questions: 1) “Do you provide information about influenza vaccination to pregnant women?” and 2) “Do you recommend influenza vaccination during pregnancy?”.
Also, the survey sought to determine each physician’s own influenza vaccination status during the previous flu season. Physicians were asked about influencing factors for future recommendations for influenza vaccination for pregnant women. Ethical approval was granted by the Institutional Review Board of The Catholic University of Korea (KC19QES10646).
Sample size calculation
The sample size for the survey of pregnant or postpartum women was calculated with the following assumptions: the proportion of women having received the influenza vaccine during pregnancy was 50%, with a confidence interval of 95% and an alpha of 0.05. The initial calculated minimum sample size was 384 participants. Given the nonresponse rate (10%) and the incomplete responses rate (30%), however, 538 pregnant women were recruited to meet the minimum sample size. The sample size for the survey of OBGYN doctors was calculated based on the estimation that 60% of OBGYN doctors routinely recommend influenza vaccination to pregnant women, with a 95% confidence interval and an alpha of 0.05. Therefore, the minimum number of required OBGYN doctors was estimated at 360. Given the 55% response rate, however, the minimum number of participants required was estimated as 640. Data collection was stopped when the minimum numbers of responses for the analyses were reached.
Data analysis
We performed all data analyses using SPSS (version 24.0; SPSS Inc., Chicago, IL, USA). Continuous variables of age of respondents and gestational weeks of pregnant women were presented as mean ± standard deviation and compared using Student’s t-test. All other variables were categorical data, which were expressed as number (%) and compared using the Chi-square test. To assess influencing factors associated with vaccine uptake by pregnant women, we calculated the odds ratios (ORs) and 95% CIs using log-binomial regression models. Variables with a significant cutoff, p < 0.2, between vaccinated and unvaccinated groups in univariate analyses were included in multivariate analyses, after adjustment for maternal age, residence, education, and occupation. Univariate analysis identified variables (p < 0.2) with OBGYN doctors’ routine recommendations for influenza vaccination between the routine and passive recommendation groups. After adjustment for physician age, sex, and location of clinic or hospital, significant variables were identified in multivariate analyses. Statistical significance was set at p < 0.05.