Key words
disease transmission, infectious influenza A virus, H1N1 subtype Orthomyxoviridae sensitivity and specificity virus shedding
In April 2009, novel influenza A/H1N1 virus (NIV) infections were first observed in Mexican and US American patients (1). Only weeks later, the virus had spread worldwide and, on June 11th, the World Health Organization announced phase 6 of pandemic alertness and thus the start of the 2009 influenza pandemic (2).
The basic clinical and epidemiologic characteristics of newly emerging pandemic viruses need to be assessed quickly, as these have implications on preventive strategies, clinical diagnosis, and epidemiologic modeling (3, 4). Regarding seasonal influenza, much of the knowledge on clinical manifestation and shedding was gained through experimental studies. Recently, a review of these studies has been published (5). Household studies represent another efficient study type that was used to examine basic influenza parameters in seasonal influenza, such as the serial interval (6), the duration of infectiousness (7), susceptibility and infectiousness of children versus adults (8), and the therapeutic and prophylactic effectiveness of neuraminidase inhibitors (9–13).
For NIV, some preliminary information, based on early—albeit limited—data, was already available, including generation time (i.e., duration of time between becoming infected and transmitting the virus to another person: 1.9 days (14) and 2.5–3 days (15)), serial interval (2.5 days (16), 2.9 days (17), 3 days (18), 4–5 days (19)), secondary household attack rates in contacts without antiviral prophylaxis (7.6% (18) and 26% (20)), and an apparently increased susceptibility of infection for children compared with adults (16–18).
We conducted a prospective, observational study of cases with NIV infection and their household contacts in Germany during the summer of 2009, in order to contribute information to the clinical and epidemiologic characteristics mentioned above.
The present study had been prepared and piloted in the interpandemic era as part of pandemic planning with the goal to rapidly collect important information on the properties of the pandemic virus upon the occurrence of the first cases in Germany.
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MATERIALS AND METHODS
When the first laboratory-confirmed pandemic cases were reported to the Robert Koch Institute, we immediately attempted to contact all case households via their respective state and local health departments to obtain verbal consent to conduct the study in the individual household. With a positive answer, the Robert Koch Institute sent out epidemiologic teams to the households of the confirmed cases. Written informed consent was obtained from all patients and their household members before enrollment. Patients and household members were visited either at home or, if hospitalized, in hospital. Enrollment lasted from April to August of 2009.
Case definition
The household index case was defined as the first person in the household with laboratory-confirmed NIV infection or as the first person with respiratory symptoms (without laboratory confirmation) when the disease of another person in the household was a laboratory-confirmed NIV infection. Household secondary cases (with disease onset at least 12 hours after the index case) had to be laboratory confirmed.
Influenza-like illness was defined as fever plus “cough or sore throat.” All participants aged less than 14 years were defined as children, and all older participants, as adults. Laboratory confirmation was defined as a positive result of any specimen tested for NIV by reverse transcriptase-polymerase chain reaction (RT-PCR). We used the term “viral shedding” when RT-PCR detected viral RNA in a nasopharyngeal specimen. A household was defined as a domestic unit consisting of the members of a family who live together including nonrelatives and intimate partners (modified according to an online dictionary (21)). Participants living in one household with the respective index patient were termed “household members” or “household contacts.”
Data and specimen collection
We assessed symptoms and antiviral medication from each household member on a daily basis starting from the day of symptom onset of the index case. Early in the study, we obtained the following specimens once a day from all household members in the following order: 1) nasal swab, 2) throat swab, 3) sputum, 4) throat wash, and 5) nasal wash. As it could only rarely be obtained from the participants, sputum was collected only occasionally. Later in the study, when the first results regarding the sensitivity of specimen types suggested a higher sensitivity for nasal wash, we took only nasal wash as a regular specimen. For the collection of nasal and throat wash, we used 5 mL of isotonic saline, which were instilled into the nostril or the mouth, respectively, and afterwards collected in a sterile cup (22). Nasal and throat swabs were collected by using virus transport swabs (Mastaswab; MAST Diagnostica, Reinfeld, Germany). Sputum was obtained after a deep cough and collected in a sterile cup. Samples were stored refrigerated (at a temperature of approximately 5°C) and analyzed as soon as possible.
Secondary attack rates
Secondary household attack rates (SARs) were calculated as the proportion of household contacts without antiviral postexposure prophylaxis becoming a case within 8 days after symptom onset of the index case. Households were excluded from calculation of the secondary household attack rate if the index case was isolated in the hospital during the period of time relevant for virus transmission. We recorded the date of the beginning of treatment (if any) and regarded treatment with neuraminidase inhibitors as “timely” if it began within the first 3 days of illness, that is, the day of symptom onset plus 2 days. As antiviral prophylaxis was not initiated according to a predefined protocol (as would be the case in a randomized controlled trial), the effectiveness of antiviral prophylaxis could not be calculated.
Serial interval
We defined the serial interval as the number of days between symptom onset of the household index case and symptom onset of the first secondary household case. Other secondary household cases were only included in the calculation of the serial interval if their symptom onset occurred on the same day as the first secondary household case.
Sensitivity of specimen types
For the calculation of sensitivity of specimen types, a sampling day was called positive on the basis of 2 conditions: First, all 4 regularly obtained specimens (nasal swab, throat swab, throat wash, nasal wash) were collected on that day, and second, at least 1 of those 4 specimens tested positive on that day. This led to 28 positive sampling days, which served as the denominator. The numerator changed with each specimen type and was defined as the number of positive results in each specimen type. The sensitivity of a specimen type was expressed in percent.
Viral shedding, viral shedding profile, and symptom profile
The minimal duration of viral shedding in symptomatic patients was defined as the time between symptom onset and the last day a RT-PCR–positive specimen was taken. For the calculation of the duration of shedding, we excluded asymptomatic study participants and those whose specimens were negative in our laboratory.
In order to display summary curves of clinical symptoms over the course of illness, we calculated a daily symptom severity score on a 4-level scale from 0 (not present) to 3 (severe) for each of the following symptoms: fever/chills, cough, sore throat, and headache/myalgia. Thus, the daily score ranged from 0 points (no symptoms) to 12 points (all symptoms with maximum severity). If, at the end of the follow-up period, symptom data were missing but the last recorded score was 0, we assumed that the respective patient continued to be asymptomatic.
For the assessment of the viral shedding profile, we included participants who were laboratory confirmed and had a definite day of symptom onset. If one or more positive test results were followed by a final negative test result, we assumed that the following days also were laboratory negative. To compare viral load (expressed as the log10 of RNA copies/mL) with the symptom score, we performed 2 calculations: 1) pooled analysis (for each illness day, we plotted the median of the log10 of RNA copies/mL against the median of symptom scores); 2) individual analysis