We identified 110 persons with laboratory-confirmed pandemic (H1N1) 2009 infection. We were unable to contact 23 (21%) of these persons, and 3 (3%) did not agree to provide further information. Of 84 persons with laboratory-confirmed pandemic (H1N1) 2009 infection who provided information, 77 (92%) lived with >1 persons. These 77 households comprised 349 persons; the median household size was 4 persons (range 2–9 persons), including the index case-patient. Seventy five percent of household interviews were conducted >8 days (range 0–24 days) after the onset of infection in the index case-patient.

From household interviews, we identified an additional 47 persons who reported respiratory symptoms or had laboratory evidence of pandemic (H1N1) 2009 infection: 13 persons with laboratory-confirmed pandemic (H1N1) 2009 infection, 24 persons with ILI, and 10 persons whose illness met the case definition for ARI only. We did not classify 15 of these persons as secondary case-patients: 8 persons had the same date of symptom onset as the index case-patient; we could not establish the date of symptom onset for 3 persons; and 4 persons reported illness onset 10–15 days after the index case-patient. In 1 household where 2 persons had ILI, 1 had a nasopharyngeal swab that was positive for pandemic (H1N1) 2009; the other was positive for influenza A, but the subtype could not be determined, possibly because of the quality of the sample or because 9 days had elapsed between illness and sample collection, thus decreasing viral load. We considered this person to have laboratory-confirmed pandemic (H1N1) 2009 on the basis of an epidemiologic link to another laboratory-confirmed case. In 2 households where secondary case-pateints were identified, nasal swab samples were obtained from members of all 7 households; 1 person, 14 years of age, who did not report any respiratory symptoms, was positive for pandemic (H1N1) 2009 infection.

Among the 97 symptomatic laboratory-confirmed case-patients (84 identified through case finding and 13 through household investigation), illness onset dates ranged from April 11 through May 8, 2009. Eleven (11%) were <4 years of age, 61 (63%) 5–18 years of age, 22 (23%) 19–49 years of age, and 3 (3%) >50 years of age. Forty-six (47%) were male. The most common signs and symptoms were fever (93%), cough (91%), rhinnorhea (70%), headache (67%), and sore throat (58%). Vomiting was reported by 26% and diarrhea by 25%. Ninety-two percent of laboratory-confirmed case-patients met the definition for ARI, and 85% met the definition for ILI. One laboratory-confirmed case-patient was hospitalized: a child who was admitted to the hospital with pneumonia in early April. No deaths occurred. Compared with household contacts who did not have laboratory-confirmed pandemic (H1N1) 2009 or did not report respiratory illness, laboratory-confirmed case-patients (index and secondary) were significantly younger (median age 17 vs. 24 years; p<0.001).

Secondary case-patients were found in 24 (31%) of 77 households; 5 had 2 secondary case-patients, and 1 had 3 case-patients (Table 1). Secondary infections appeared most likely to be transmitted between children (12/32, 38%) or children to adults (10/32, 31%) than from adults to children (6/32, 19%) or adults to adults (4/32, 13%) (p = 0.034). The median serial interval for ARI, ILI, and laboratory-confirmed pandemic (H1N1) 2009 combined was 4 days (range 1–9 days) (Table 1; Figure A1). Antiviral treatment was given to the index case-patient of 23 (72%) of 32 secondary case-patients; in these households, the serial interval was 3 days, compared with 5 days when the index case-patient was not given treatment (p = 0.17). Inclusion of 5 household contacts with illness that occurred 10–15 days after symptom onset of the index case-patient did not alter the median serial interval estimate. The median serial interval also remained unchanged when only members of households interviewed >9 days after the onset of symptoms in the household index case-patient were included. Limiting the estimate of median serial interval to include only persons with ILI or laboratory-confirmed case-patients reduced the median serial interval to 3 days (range 1–8 days).

The secondary household attack rate was 13% for ARI, 9% for ILI, and 4% for laboratory-confirmed pandemic (H1N1) 2009 (Table 2). Secondary attack rates were highest in children <5 years of age and were higher in children 5–18 years of age than in adults 19–49 and >50 years of age (Table 2). By household size, secondary attack rates for ARI, ILI, and laboratory-confirmed pandemic (H1N1) 2009 were highest in households with 2–3 persons (ARI 23%, ILI 23%, laboratory-confirmed pandemic [H1N1] 2009 6%) and were lowest in households with 7–9 persons (ARI 9%, ILI 9%, laboratory-confirmed pandemic [H1N1] 2009 2%) (Table A1). The secondary household attack rate did not differ by receipt of seasonal influenza vaccination in the previous 12 months (Table A2).

Treatment with antiviral medication was given to 77% of index case-patients (57/74 of persons with ARI, ILI, and laboratory-confirmed pandemic [H1N1] 2009 combined for whom data were available) and 72% of secondary cases (23/32 of ARI, ILI, and laboratory-confirmed pandemic [H1N1] 2009 combined); 90% took oseltamivir; 7% took zanamivir; and 3% took an unknown type of antiviral medication. Neither the age of the index case-patient, household size, nor diagnosis of the index patient (with ARI, ILI, or laboratory-confirmed pandemic [H1N1] 2009) were predictive of treatment with antiviral medication. The secondary household attack rates for ARI, ILI, and laboratory-confirmed pandemic (H1N1) 2009 combined in households where the index case-patient was given antiviral treatment was 12% compared with 16% in other households (p = 0.64) (Table 3). Antiviral prophylaxis was given to 39% of household contacts (92/235 with data available) (Table 3), and the secondary attack rate of ARI, ILI, and laboratory-confirmed pandemic (H1N1) 2009 combined was 14% (12/83) in households where the index patient took treatment, compared with 66% (6/9) (p = 0.003) in households where the index patient did not take treatment (Table 3).

During an outbreak of pandemic (H1N1) 2009 in the San Antonio, Texas, area, we identified 97 persons with laboratory-confirmed infection in 77 households. The epidemiologic and clinical features were similar to summary reports from the United States (14,15) and other countries (15,16). Nearly one third of households had secondary case-patients who also had respiratory illness, with a median of 4 days between onset of illness in the index case-patient and household members, a finding similar to that for seasonal influenza (17).

The secondary attack rate was 4% for laboratory-confirmed pandemic (H1N1) 2009, 9% for ILI, and 13% for ARI. In general, these rates are lower than for seasonal influenza and lower than anticipated for a pandemic strain, although rates vary from 13% to 30%, depending on influenza subtype and year and pandemic period (4,5,18–21). The highest proportion of laboratory-confirmed pandemic (H1N1) 2009 and secondary attack rates occurred in children, a finding consistent with the epidemiology of seasonal and pandemic influenza, where we know children experience higher rates of illness (4,5,7) and higher secondary attack rates (19). Adults may have some cross-protection against pandemic (H1N1) 2009 from antibodies developed during infections with seasonal influenza A virus (H1N1) (22–24).

Four randomized controlled trials of zanamivir and oseltamivir for seasonal influenza have shown that these antiviral medications reduce but do not eliminate viral shedding and are effective in preventing disease among household contacts, especially if taken within 48 hours of illness onset in the index case-patient (19,20,25,26). We found that secondary attack rates for all households were lower when the index case-patient received treatment, although this difference was not significant. The role of prophylaxis in the absence of treatment of the index case-patient was difficult to determine; our investigation included only a small number of such persons. Nevertheless, because most index and secondary case-patients received antiviral treatment, household secondary attack rates may have been reduced.

Our investigation has several limitations. Because early case finding was most intensive among high school children associated with school outbreaks, our cohort may have been biased in favor of households where the index case-patients were children; however, this would not explain a lower secondary attack rate among adult household contacts. We did not assess the role of mild or asymptomatic pandemic (H1N1) 2009 infection because we collected respiratory samples only; serologic assays to detect influenza antibodies are the most sensitive method for detecting asymptomatic infection, but virus assays for pandemic (H1N1) 2009 were not available at the time of the investigation. Volunteer challenge studies with seasonal influenza viruses have found that up to 30% of infected persons may be asymptomatic and could be identified through serologic testing (10). Because 25% of household interviews were conducted <8 days after onset of illness of the index case-patient, we may have underestimated the secondary attack rate if these households had secondary case-patients with long serial intervals. However, when we restricted our analysis to persons interviewed >8 days after onset of symptoms in the index case-patient, we found no difference in the median serial interval or distribution of attack rates by age. Conversely, household members interviewed >8 days after onset of illness in the index case-patient may have had incomplete recall of acute respiratory infections. Finally, some of the secondary illnesses may have been acquired in the community, leading to overestimate of household secondary attack rates.

We found that pandemic (H1N1) 2009 disproportionately affected children, who in turn posed a risk for secondary household transmission, especially to their caregivers and siblings. The Advisory Committee on Immunization Practices (2009) recommends that children 6–18 years of age and caregivers of infants be included as initial target groups for the new pandemic (H1N1) 2009 vaccine (27), which may reduce household transmission. As pandemic (H1N1) 2009 continues to spread internationally, ongoing investigations are needed to shed further light on transmission dynamics, to monitor epidemiologic changes over time, and to assess the effectiveness of public health interventions.