We report an outbreak of 97 H1N1 cases among military beneficiaries in San Diego County from 21 April through 8 May 2009. The outbreak involved persons of all age groups, but it mostly affected young adults. Active duty members of the military were also preferentially affected; 25% of the beneficiary population is on active duty, yet this group accounted for 62% of cases of H1N1 virus infection.
A unique outbreak occurred on a docked Navy ship (ship A), in which 8% of the 402 crew members developed confirmed H1N1 virus infection. Using detailed data on the crew and their family members (crew members returned home each night to their families), we estimated the secondary attack rate as 6%–14%. Although this number may underestimate the secondary attack rate because of the possibility of cases not presenting for care, medical care was readily available onboard the ship, and ill beneficiaries were encouraged to receive care. Compared with previous pandemic influenza strains [18
], this novel H1N1 strain may have lower or similar rates of transmissibility; however, unlike the swine flu outbreak that occurred at Fort Dix in 1976 [20
], it has resulted in sustained human-to-human transmission.
Fever and cough were more commonly seen in patients with H1N1 virus infection than in those with noninfluenza diagnoses. These symptoms are highly predictive for the diagnosis of seasonal influenza during outbreaks [13
]. Both symptoms are part of the CDC case definition for ILI; however, H1N1 testing was performed in a nondifferential fashion among persons presenting with an ILI. Nonetheless, it is possible that cases without case-defining symptoms, such as cases of nonfebrile H1N1 virus infection, were missed. Patients with H1N1 virus infection were also more likely to report myalgia or arthralgia and headache, but they were less likely to have diarrhea, compared with individuals with noninfluenza ILI. The clinical course of H1N1 virus infection was generally mild: the percentage of patients with concurrent pneumonia was 2%, the percentage with hospitalization was 1%, and there were no fatalities, suggesting that this novel influenza strain usually does not lead to severe disease, similar to seasonal strains [22
], notwithstanding recent unfortunate reports of fatal H1N1 virus infections. Although our population was similar to the local community, it consisted of slightly younger persons (median age, 29 vs 33 years; 41% vs 37% were <25 years of age) [24
] and may have included healthier individuals, potentially contributing to the low rates of severe disease in our cohort.
Medical conditions, such heart disease or diabetes, were not associated with H1N1 virus infection. This may be attributable to the low number of H1N1 virus infections among older age groups. We found that pregnancy was associated with H1N1 virus infection, although our numbers were small. Reports suggest that pregnant women with H1N1 virus infection are at higher risk for hospitalization, obstetric complications, and death [25
], as are those with seasonal influenza [22
]. These data emphasize the importance of assessing the safety and efficacy of H1N1 vaccines among pregnant women.
H1N1 virus infection preferentially affected adolescents and young adults. These trends differ from those for seasonal influenza viruses, which have the greatest impact on individuals at the extremes of age [22
]. Of note, the 1918–1919 strain also primarily affected young adults [28
]. Some of the predisposition of young age groups may have been influenced by cohorting of young active duty members onboard ships or at training depots; however, the lower age-specific incidence rates among individuals <2 years of age and >50 years of age suggests that H1N1 virus infection preferentially affects adolescents and young adults.
We noted trends in an increased number of cases among black individuals. We are unaware of data suggesting that any influenza preferentially involves specific ethnic groups. The nature of this association in our study is unknown, but it may reflect a statistical anomaly, clustering among black individuals in workplaces and neighborhoods, or unknown host genetic factors [30
]. Of note, both age and racial trends remained even when active duty members were excluded, among whom case clustering is more likely to occur.
Seasonal influenza vaccination was not protective against H1N1 virus; this was expected, because recent data showed that seasonal influenza vaccination did not induce protective antibody against H1N1 virus, and there was no significant serologic cross-reactivity [31
]. Active duty members of the military with H1N1 virus infection (but no other group) were more likely to have received influenza vaccination (particularly LAIV) than were those without H1N1 virus infection. This finding is likely explained by vaccination of nearly all military members, and it is likely not a true association. A vaccine specifically effective against H1N1 virus infection is currently under production [32
Regarding the diagnosis of novel H1N1 virus infections, the RIAT used in this study had low sensitivity but similar specificity for detecting H1N1, compared with its sensitivity and specificity for detecting seasonal influenza strains [33
]. The RIAT performed best among individuals <18 years of age; this may be related to increased viral shedding among children, compared with that among adults, as seen with seasonal influenza [34
]. Our results emphasize the importance of rRT-PCR in diagnosing novel H1N1 virus infection. Potential limitations of our investigation included the fact that not all ILI cases may have presented for care or received rRT-PCR testing, leading to an underestimation of the number of cases. Viral culture was not performed, because the virus does not grow well in standard respiratory tissue cultures; culture of H1N1 virus was initially restricted; and rRT-PCR appears to be more sensitive than viral culture [13
]. We only evaluated individuals with H1N1 virus infection who were eligible for care in our system; consequently, we may have missed linkages between our beneficiaries and other community cases. Conversely, some beneficiaries may have received a diagnosis in the community, particularly those individuals >65 years of age; however, medical services are at no cost for beneficiaries at our facilities. Although a large number of H1N1 virus infections among active duty members could potentially bias some of our comparisons between H1N1 virus infection and noninfluenza cases, we analyzed our data stratified by duty status and found similar associations except for prior influenza vaccination and male sex. Finally, despite a robust EMR, some clinical data (eg, physicians’ handwritten notes and immunization records) were not available during our investigation.
Strengths included our well-defined population, comprising all age groups, with a diverse racial composition. We used detailed electronic information regarding demographic data, residency, medical encounters, and vaccinations to comprehensively describe the characteristics of the novel H1N1 strain. In addition, our affiliated laboratory center (NHRC) had rapid throughput rRT-PCR capability. Finally, our active duty population has unique exposure environments, such as ships and training depots, which illustrated the impact of H1N1 virus infection.
In summary, we report the clinical and epidemiologic features of an H1N1 virus infection outbreak among military beneficiaries in San Diego, California. The strain appears to preferentially affect adolescents and young adults, with pregnant women also having a greater risk of clinical infection. Although most cases resulted in a mild, febrile illness without sequelae, our investigation demonstrates the rapid spread of this novel H1N1 strain in a well-defined population, including an outbreak on a military vessel.