Estimation of the Basic Reproductive Number and Mean Serial Interval of a Novel Pathogen in a Small, Well-Observed Discrete Population.

BACKGROUND: Accurately assessing the transmissibility and serial interval of a novel human pathogen is public health priority so that the timing and required strength of interventions may be determined. Recent theoretical work has focused on making best use of data from the initial exponential phase of growth of incidence in large populations. METHODS: We measured generational transmissibility by the basic reproductive number R0 and the serial interval by its mean Tg. First, we constructed a simulation algorithm for case data arising from a small population of known size with R0 and Tg also known. We then developed an inferential model for the likelihood of these case data as a function of R0 and Tg. The model was designed to capture a) any signal of the serial interval distribution in the initial stochastic phase b) the growth rate of the exponential phase and c) the unique combination of R0 and Tg that generates a specific shape of peak incidence when the susceptible portion of a small population is depleted. FINDINGS: Extensive repeat simulation and parameter estimation revealed no bias in univariate estimates of either R0 and Tg. We were also able to simultaneously estimate both R0 and Tg. However, accurate final estimates could be obtained only much later in the outbreak. In particular, estimates of Tg were considerably less accurate in the bivariate case until the peak of incidence had passed. CONCLUSIONS: The basic reproductive number and mean serial interval can be estimated simultaneously in real time during an outbreak of an emerging pathogen. Repeated application of these methods to small scale outbreaks at the start of an epidemic would permit accurate estimates of key parameters.

Effect on Baby-Friendly Hospital Steps When Hospitals Implement a Policy to Pay for Infant Formula.

BACKGROUND: The Baby-Friendly Hospital Initiative requires hospitals to pay market price for infant formula. No studies have specifically examined the effect of hospitals paying for infant formula on breastfeeding mothers' exposure to Baby-Friendly steps. OBJECTIVES: To investigate the effect of hospitals implementing a policy of paying for infant formula on new mothers' exposure to Baby-Friendly steps and examine the effect of exposure to Baby-Friendly steps on breastfeeding rates. METHODS: We used a repeated prospective cohort study design. We recruited 2 cohorts of breastfeeding mother-infant pairs (n = 2470) in the immediate postnatal period from 4 Hong Kong public hospitals and followed them by telephone up to 12 months postpartum. We assessed participants' exposure to 6 Baby-Friendly steps by extracting data from the medical record and by maternal self-report. RESULTS: After hospitals began paying for infant formula, new mothers were more likely to experience 4 out of 6 Baby-Friendly steps. Breastfeeding initiation within the first hour increased from 28.7% to 45%, and in-hospital exclusive breastfeeding rates increased from 17.9% to 41.4%. The proportion of mothers who experienced all 6 Baby-Friendly steps increased from 4.8% to 20.5%. The risk of weaning was progressively higher among participants experiencing fewer Baby-Friendly steps. Each additional step experienced by new mothers decreased the risk of breastfeeding cessation by 8% (hazard ratio = 0.92; 95% CI, 0.89-0.95). CONCLUSION: After implementing a policy of paying for infant formula, breastfeeding mothers were exposed to more Baby-Friendly steps, and exposure to more steps was significantly associated with a lower risk of breastfeeding cessation.

Effect of a hospital policy of not accepting free infant formula on in-hospital formula supplementation rates and breast-feeding duration.

OBJECTIVE: To investigate the effect of public hospitals in Hong Kong not accepting free infant formula from manufacturers on in-hospital formula supplementation rates and breast-feeding duration. DESIGN: Prospective cohort study. SETTING: In-patient postnatal units of four public hospitals in Hong Kong. SUBJECTS: Two cohorts of breast-feeding mother-infant pairs (n 2560). Cohort 1 (n 1320) was recruited before implementation of the policy to stop accepting free infant formula and cohort 2 (n 1240) was recruited after policy implementation. Participants were followed prospectively for 12 months or until they stopped breast-feeding. RESULTS: The mean number of formula supplements given to infants in the first 24 h was 2·70 (sd 3·11) in cohort 1 and 1·17 (sd 1·94) in cohort 2 (P<0·001). The proportion of infants who were exclusively breast-fed during the hospital stay increased from 17·7 % in cohort 1 to 41·3 % in cohort 2 (P<0·001) and the risk of breast-feeding cessation was significantly lower in cohort 2 (hazard ratio=0·81; 95 % CI 0·73, 0·90). Participants who non-exclusively breast-fed during the hospital stay had a significantly higher risk of stopping any or exclusive breast-feeding. Higher levels of formula supplementation also increased the risk of breast-feeding cessation in a dose-response pattern. CONCLUSIONS: After implementation of a hospital policy to pay market price for infant formula, rates of in-hospital formula supplementation were reduced and the rates of in-hospital exclusive breast-feeding and breast-feeding duration increased.

Simulation-guided design of serological surveys of the cumulative incidence of influenza infection.

BACKGROUND: Influenza infection does not always cause clinical illnesses, so serological surveillance has been used to determine the true burden of influenza outbreaks. This study investigates the accuracy of measuring cumulative incidence of influenza infection using different serological survey designs. METHODS: We used a simple transmission model to simulate a typical influenza epidemic and obtained the seroprevalence over time. We also constructed four illustrative scenarios for baseline levels of antibodies prior and levels of boosting following infection in the simulated studies. Although illustrative, three of the four scenarios were based on the most detailed empirical data available. We used standard analytical methods to calculate estimated seroprevalence and associated confidence intervals for each of the four scenarios for both cross-sectional and longitudinal study designs. We tested the sensitivity of our results to changes in the sampled size and in our ability to detect small changes in antibody levels. RESULTS: There were substantial differences between the background antibody titres and levels of boosting within three of our illustrative scenarios which were based on empirical data. These differences propagated through to different and substantial patterns of bias for all scenarios other than those with very low background titre and high levels of boosting. The two survey designs result in similar seroprevalence estimates in general under these scenarios, but when background immunity was high, simulated cross-sectional studies had higher biases. Sensitivity analyses indicated that an ability to accurately detect low levels of antibody boosting within paired sera would substantially improve the performance of serological surveys, even under difficult conditions. CONCLUSIONS: Levels of boosting and background immunity significantly affect the accuracy of seroprevalence estimations, and depending on these levels of immunity responses, different survey designs should be used to estimate seroprevalences. These results suggest that under current measurement criteria, cumulative incidence measured by serological surveys might have been substantially underestimated by failing to include all infections, including mild and asymptomatic infections, in certain scenarios. Dilution protocols more highly resolved than serial 2-fold dilution should be considered for serological surveys.

Social contacts and the locations in which they occur as risk factors for influenza infection.

The interaction of human social behaviour and transmission is an intriguing aspect of the life cycle of respiratory viral infections. Although age-specific mixing patterns are often assumed to be the key drivers of the age-specific heterogeneity in transmission, the association between social contacts and biologically confirmed infection has not previously been tested at the individual level. We administered a questionnaire to participants in a longitudinal cohort survey of influenza in which infection was defined by longitudinal paired serology. Using a variety of statistical approaches, we found overwhelming support for the inclusion of individual age in addition to contact variables when explaining odds of infection: the best model not including age explained only 15.7% of the deviance, whereas the best model with age explained 23.6%. However, within age groups, we did observe an association between contacts, locations and infection: median numbers of contacts (or locations) reported by those infected were higher than those from the uninfected group in every age group other than the youngest. Further, we found some support for the retention of location and contact variables in addition to age in our regression models, with excess odds of infection of approximately 10% per additional 10 contacts or one location. These results suggest that, although the relationship between age and incidence of respiratory infection at the level of the individual is not driven by self-reported social contacts, risk within an age group may be.

Factors contributing to early breast-feeding cessation among Chinese mothers: an exploratory study.

BACKGROUND: Although more than 85% of all new mothers in Hong Kong now initiate breast feeding, few exclusively breast feed and the overall duration is short. More than one-third stop breast feeding within the first month post partum. OBJECTIVE: To explore the breast-feeding experiences of Hong Kong Chinese mothers who prematurely discontinue breast feeding and to identify contributing factors that might be remediated to help women breast feed longer. DESIGN: Qualitative exploratory study. METHODS: In-depth, exploratory interviews were carried out with 24 new mothers who stopped breast feeding within one month after birth, and content analysis was used to analyse the data. FINDINGS: Five core themes emerged from the data: unnatural expectations, left to figure it out, uncertainty, unfulfilling experiences, and guilt versus relief. Because breast feeding is 'natural' participants expected that it would come naturally and thus be easy. When breast feeding did not happen naturally, however, midwives were too busy to provide breast-feeding support and mothers were left to figure it out on their own. Participants also reported difficulty in gauging whether the infant was getting adequate nutrition from their breastmilk. Few participants had positive breast-feeding experiences; while the decision to stop breast feeding caused guilt for most participants, others expressed relief at stopping breast feeding. KEY CONCLUSIONS AND IMPLICATIONS FOR PRACTICE: Greater postnatal breast-feeding support, both in the hospital and after the mother returns home, would likely increase the mother׳s confidence and enhance her mothering experience. Further antenatal and postnatal education on the realistic breast-feeding expectations and the amount of breastmilk required by babies is also important. More research is needed to test professional and peer support breast-feeding interventions to provide guidance to policy makers on the most effective breast-feeding support strategies.

Predictors and consequences of in-hospital formula supplementation for healthy breastfeeding newborns.

BACKGROUND: Although exclusive breastfeeding is recommended for the first 6 months, the use of breast milk substitutes is widespread around the world. OBJECTIVES: To describe the patterns of infant formula supplementation among healthy breastfeeding newborns, to identify factors contributing to in-hospital formula supplementation, and to assess the dose-response relationship between the amount of in-hospital formula supplementation and the duration of any breastfeeding. METHODS: A sample of 1246 breastfeeding mother-infant pairs was recruited from 4 public hospitals in Hong Kong and followed prospectively for 12 months or until weaned. Multiple logistic regression analysis was used to examine factors associated with in-hospital supplementation. Cox regression analysis was used to explore the impact of in-hospital supplementation on breastfeeding duration. RESULTS: Of the total, 82.5% of newborns were supplemented in the hospital; one-half received formula within 5 hours of birth. Assisted vaginal delivery (odds ratio [OR] = 2.06, 95% confidence interval [CI] 1.03, 4.15), cesarean section (OR = 3.45, 95% CI 1.75, 6.80), and higher birth weight (OR = 1.56, 95% CI 1.12, 2.18) were positively associated with in-hospital formula supplementation, whereas initiating breastfeeding in the delivery room (OR = 0.55, 95% CI 0.33, 0.89) was associated with decreased likelihood of in-hospital supplementation. Any infant formula in the first 48 hours was associated with a shorter duration of breastfeeding (hazard ratio [HR] = 1.51, 95% CI 1.27, 1.80), but there was no dose-response effect. CONCLUSION: In-hospital formula supplementation is common in Hong Kong hospitals and appears to be detrimental to breastfeeding duration. Continued efforts should be made to avoid the provision of infant formula to breastfeeding babies while in the hospital unless medically indicated.

Association between intrapartum interventions and breastfeeding duration.

INTRODUCTION: Few women who reside in Hong Kong exclusively breastfeed, and one-half stop breastfeeding within the first few months. There is little research in this population on the association between intrapartum interventions and breastfeeding duration. METHODS: A sample of 1280 mother-infant pairs were recruited from the obstetric units of 4 public hospitals in Hong Kong and followed prospectively for 12 months or until the infant was weaned. The outcome variables for this analysis were the duration of any and exclusive breastfeeding. Predictor variables were 4 intrapartum interventions: receipt of opioid pain medication, induction versus spontaneous labor, epidural administration, and mode of birth. We used Cox proportional hazards modeling to assess the impact of intrapartum interventions on the duration of any and exclusive breastfeeding, and we constructed Kaplan-Meier survival curves to evaluate the cumulative impact of multiple intrapartum interventions on breastfeeding outcomes. RESULTS: Bivariate analysis showed that induction of labor (hazard ratio [HR], 1.24; 95% confidence interval [CI], 1.09-1.41), opioid pain medication (HR, 1.21; 95% CI, 1.06-1.37), and having an emergency cesarean birth (HR, 1.22; 95% CI, 1.01-1.48) were associated with a shorter duration of any breastfeeding. Induction of labor (HR, 1.23; 95% CI, 1.08-1.39) and having an emergency cesarean birth (HR, 1.25; 95% CI, 1.05-1.51) were associated with a shorter duration of exclusive breastfeeding. After controlling for known confounding variables, there was no longer any association between individual intrapartum interventions and the duration of any or exclusive breastfeeding. The median duration of breastfeeding for participants who experienced a natural birth with no intrapartum interventions was 9 weeks compared with 5 weeks for participants who experienced at least 3 intrapartum interventions. DISCUSSION: Clinicians working with new breastfeeding mothers should focus on providing additional support to mothers who experience a difficult labor and birth with multiple interventions to improve their breastfeeding experiences.

Determinants of 2009 A/H1N1 influenza vaccination among pregnant women in Hong Kong.

During the 2009­2010 A/H1N1 influenza pandemic, pregnant women infected with the virus experienced excess morbidity and mortality when compared with other groups. Once a vaccine was available, pregnant women were a priority group for vaccination. Only a few studies have reported on the uptake of 2009 A/H1N1 influenza vaccine among pregnant women during the pandemic and none were from Asia. The purpose of this study was to examine factors associated with 2009 A/H1N1 influenza vaccine uptake among pregnant women in Hong Kong. Using a multi-center, cross-sectional design, we recruited 549 postpartum women from four post-natal wards in Hong Kong over a 4-month period during the second wave of the A/H1N1 influenza pandemic in the winter and spring of 2010. Only 6.2% (n = 34) of participants had received the 2009 A/H1N1 influenza vaccine and 4.9% (n = 27) had received the seasonal influenza vaccine. The most common reasons for not receiving the 2009 A/H1N1 vaccine were fear of causing harm to themselves or their fetus. A high knowledge level (OR = 19.06; 95% CI 5.55, 65.48), more positive attitudes (OR = 3.52; 95% CI 1.37, 9.07), and having a family member who had the 2009 A/H1N1 influenza vaccine (OR = 7.69; 95% CI 2.92, 20.19) were independently and positively associated with vaccination. Study results show an unacceptably low uptake of the pandemic A/H1N1 influenza vaccine among pregnant women in Hong Kong. Interventions to increase influenza vaccine knowledge and uptake among this group should be a priority for future pandemic planning and seasonal vaccination campaigns.

Impact of baby-friendly hospital practices on breastfeeding in Hong Kong.

BACKGROUND: The World Health Organization (WHO) developed the Baby-Friendly Hospital Initiative to improve hospital maternity care practices that support breastfeeding. In Hong Kong, although no hospitals have yet received the Baby-Friendly status, efforts have been made to improve breastfeeding support. The aim of this study was to examine the impact of Baby-Friendly hospital practices on breastfeeding duration. METHODS: A sample of 1,242 breastfeeding mother-infant pairs was recruited from four public hospitals in Hong Kong and followed up prospectively for up to 12 months. The primary outcome variable was defined as breastfeeding for 8 weeks or less. Predictor variables included six Baby-Friendly practices: breastfeeding initiation within 1 hour of birth, exclusive breastfeeding while in hospital, rooming-in, breastfeeding on demand, no pacifiers or artificial nipples, and information on breastfeeding support groups provided on discharge. RESULTS: Only 46.6 percent of women breastfed for more than 8 weeks, and only 4.8 percent of mothers experienced all six Baby-Friendly practices. After controlling for all other Baby-Friendly practices and possible confounding variables, exclusive breastfeeding while in hospital was protective against early breastfeeding cessation (OR: 0.61; 95% CI: 0.42-0.88). Compared with mothers who experienced all six Baby-Friendly practices, those who experienced one or fewer Baby-Friendly practices were almost three times more likely to discontinue breastfeeding (OR: 3.13; 95% CI: 1.41-6.95). CONCLUSIONS: Greater exposure to Baby-Friendly practices would substantially increase new mothers' chances of breastfeeding beyond 8 weeks postpartum. To further improve maternity care practices in hospitals, institutional and administrative support are required to ensure all mothers receive adequate breastfeeding support in accordance with WHO guidelines.

Epidemiological characteristics of 2009 (H1N1) pandemic influenza based on paired sera from a longitudinal community cohort study.

BACKGROUND: While patterns of incidence of clinical influenza have been well described, much uncertainty remains over patterns of incidence of infection. The 2009 pandemic provided both the motivation and opportunity to investigate patterns of mild and asymptomatic infection using serological techniques. However, to date, only broad epidemiological patterns have been defined, based on largely cross-sectional study designs with convenience sampling frameworks. METHODS AND FINDINGS: We conducted a paired serological survey of a cohort of households in Hong Kong, recruited using random digit dialing, and gathered data on severe confirmed cases from the public hospital system (>90% inpatient days). Paired sera were obtained from 770 individuals, aged 3 to 103, along with detailed individual-level and household-level risk factors for infection. Also, we extrapolated beyond the period of our study using time series of severe cases and we simulated alternate study designs using epidemiological parameters obtained from our data. Rates of infection during the period of our study decreased substantially with age: for 3-19 years, the attack rate was 39% (31%-49%); 20-39 years, 8.9% (5.3%-14.7%); 40-59 years, 5.3% (3.5%-8.0%); and 60 years or older, 0.77% (0.18%-4.2%). We estimated parameters for a parsimonious model of infection in which a linear age term and the presence of a child in the household were used to predict the log odds of infection. Patterns of symptom reporting suggested that children experienced symptoms more often than adults. The overall rate of confirmed pandemic (H1N1) 2009 influenza (H1N1pdm) deaths was 7.6 (6.2-9.5) per 100,000 infections. However, there was substantial and progressive increase in deaths per 100,000 infections with increasing age from 0.66 (0.65-0.86) for 3-19 years up to 220 (50-4,000) for 60 years and older. Extrapolating beyond the period of our study using rates of severe disease, we estimated that 56% (43%-69%) of 3-19 year olds and 16% (13%-18%) of people overall were infected by the pandemic strain up to the end of January 2010. Using simulation, we found that, during 2009, larger cohorts with shorter follow-up times could have rapidly provided similar data to those presented here. CONCLUSIONS: Should H1N1pdm evolve to be more infectious in older adults, average rates of severe disease per infection could be higher in future waves: measuring such changes in severity requires studies similar to that described here. The benefit of effective vaccination against H1N1pdm infection is likely to be substantial for older individuals. Revised pandemic influenza preparedness plans should include prospective serological cohort studies. Many individuals, of all ages, remained susceptible to H1N1pdm after the main 2009 wave in Hong Kong. Please see later in the article for the Editors' Summary.

Breastfeeding and weaning practices among Hong Kong mothers: a prospective study.

BACKGROUND: Breastfeeding provides optimal and complete nutrition for newborn babies. Although new mothers in Hong Kong are increasingly choosing to breastfeed their babies, rates of exclusive breastfeeding are low and duration remains short. The purpose of this study was to describe the breastfeeding and weaning practices of Hong Kong mothers over the infant's first year of life to determine the factors associated with early cessation. METHODS: A cohort of 1417 mother-infant pairs was recruited from the obstetric units of four public hospitals in Hong Kong in the immediate post-partum period and followed prospectively for 12 months or until weaned. We used descriptive statistics to describe breastfeeding and weaning practices and multiple logistic regression to investigate the relationship between maternal characteristics and breastfeeding cessation. RESULTS: At 1 month, 3 months, 6 months and 12 months only 63%, 37.3%, 26.9%, and 12.5% of the infants respectively, were still receiving any breast milk; approximately one-half of breastfeeding mothers were exclusively breastfeeding. Younger mothers, those with a longer duration of residence in Hong Kong, and those returning to work postpartum were more likely to wean before 1 month. Mothers with higher education, previous breastfeeding experience, who were breastfed themselves and those who were planning to exclusively breastfeed and whose husbands preferred breastfeeding were more likely to continue breastfeeding beyond 1 month. The introduction of infant formula before 1 month and returning to work postpartum were predictive of weaning before 3 months. CONCLUSIONS: Breastfeeding promotion programs have been successful in achieving high rates of breastfeeding initiation but the focus must now shift to helping new mothers exclusively breastfeed and sustain breastfeeding for longer.