Study design and population
The NINFEA study is an Italian internet-based mother-child cohort (www.progettoninfea.it) set up to investigate the influence of early-life exposures on later childhood and adulthood health. Between 2005 and 2016, approximately 7,500 pregnant women were recruited by completing the baseline questionnaire, and the children are currently followed up with questionnaires completed by mothers at 6 and 18 months after delivery and when the children turn 4, 7, 10 and 13 years. Details on the cohort have been published before [10-13].
The NINFEA study was approved by the Ethical Committee of the San Giovanni Battista Hospital and CTO/CRF/Maria Adelaide Hospital of Turin (project number 45) and all the participants gave informed consent at enrolment and after completing each study follow-up questionnaire. A specific amendment to the Ethical Committee was submitted for the COVID-19 survey. All procedures were conducted in accordance with the relevant guidelines and regulations.
COVID-19 survey
Women who completed at least the first NINFEA follow-up questionnaire (when the child was 6 months old, N=5879) were invited to complete an anonymous online questionnaire to assess the prevalence of COVID-19-like symptoms in their households. A first e-mail was sent out on April 7th 2020, approximately 5 weeks after the Italian government imposed national lockdown. The questionnaire remained open for 13 days (until April 20th) and, during this period, two reminder e-mails were sent.
The questionnaire consisted of background information on respondent’s age, sex, year of recruitment into the NINFEA cohort, educational level, province, region and area of residence, and the source from which information on COVID-19 was sought. The second part of the questionnaire asked about family composition and sex and age of family members, and included a checklist of COVID-19-like symptoms since the day of the first reported case in Italy (February 21st 2020), and in the last week, for each close family member (mother, partner, and each child <18 years old). The symptoms included: nasal congestion, low-grade fever (37.0-37.5oC), fever (>37.5oC), cough, sore throat, nausea/vomiting, diarrhoea, muscle pain, and fatigue. Questions on breathing difficulties and loss of taste or smell (anosmia/dysgeusia) were introduced a few days after launching the questionnaire, and are available for 64.2% of the respondents. We also collected information on SARS-CoV-2 testing (using nasopharyngeal swab and real-time reverse transcription polymerase chain reaction [RT-PCR]) and COVID-19 diagnosis for each of the close family members and other people living in the same household.
Administrative data
The population cumulative incidence of new SARS-CoV-2 positive cases until April 7th 2020 by province was obtained from national Surveillance System data available at the website of the Italian Ministry of Health/Civil Protection Department [14], and province population size (all residents as of January 1st 2020) obtained from the Italian National Institute of Statistics [15].
Statistical analyses
To account for survey non-response, weights for each respondent were calculated using iterative proportional fitting [16], allowing the distribution of the survey variables to closely resemble the known NINFEA population margins. The weights were calculated using the following maternal characteristics: age (<35 years, 35-40 years, 40-45 years, 45-50 years and ≥50 years), educational level (low - primary school or less, medium - secondary school, and high - university degree or higher), and period of enrolment into the NINFEA study (2005-2008, 2009-2012, 2013-2016).
Using the estimated weights, descriptive statistics were calculated for socio-demographic characteristics, cumulative symptoms, SARS-CoV-2 testing and COVID-19 diagnosis separately for children <6 years, children 6-18 years, and adults.
To explore the geographical correlation between the prevalence of COVID-19-like symptoms and the population cumulative number of new reported SARS-CoV-2 cases, we first estimated the predicted probability of each symptom given the province of residence using weighted logistic regression models and accounting for family cluster. These analyses were performed only in provinces with more than 50 study subjects (Alessandria, Asti, Cuneo and Torino, in Piedmont; Milan, in Lombardy; Arezzo, Lucca, and Florence, in Tuscany; Rome, in Lazio). The predicted probabilities were correlated to the corresponding province cumulative incidences per 1000 inhabitants (as of April 7th 2020), using Spearman’s rank correlation coefficients. These analyses were performed separately in 6-18 years old children, in adults, and at the household-level.
To analyse the clustering of symptoms within families exposed to SARS-CoV-2 we used a three-level exposure variable defined as: i) no family/household member tested for SARS-CoV-2, ii) at least one tested member but none with COVID-19, and iii) at least one member being diagnosed with COVID-19. This exposure was analysed in association with the presence of each COVID-19-like symptom, separately in 6-18 years old children and in adults. We estimated the prevalence ratios, with corresponding 95% confidence intervals (CI), using weighted Poisson regression models with cluster-robust standard errors to account for the family structure. Models were adjusted for sex, age, maternal educational level (low, medium, high), family size (2 members, 3 members, 4 members, and ≥5 members), area of residence (urban, suburban, and rural), and region of residence (Piedmont, Tuscany, Lombardy, other regions of Northern Italy, Central Italian regions, Southern Italian regions, and abroad), and maternal age (for analysis on children). A sensitivity analysis was performed by excluding all reported COVID-19 cases.
For each symptom, we calculated its sensitivity for COVID-19 among NINFEA adults and its positive (PPV) and negative (NPV) predictive values among NINFEA adults tested for SARS-CoV-2. As more than 60% of the NINFEA participants come from Piedmont, one of the most affected Italian regions by COVID-19, we repeated the analyses restricting to Piedmont residents. For these, we also estimated the population PPV of each symptom.
All analyses were conducted using Stata version 15.1 (College Station, Texas, USA).