Transmissibility substantially higher than that seen with seasonal influenza strains and may be comparable with pandemic influenza strains.
Transmission of the 2009 H1N1 influenza virus in households is lower than that in previous pandemics (Cauchemez S).
The transmission of H1N1 infection in 216 index patients and 600 household contacts revealed: 13% of household contacts developed an acute respiratory illness, 72% of household contacts had no respiratory illness, in 21% of households illness developed in 1 contact and in 6% illness developed in more than 1 contact (Cauchemez S).
Clinical severity of infection appears to be less than that seen in 1918 pandemic but comparable with that seen in 1957.
The illness seems self-limited and uncomplicated in most patients, that substantial numbers of patients have severe disease and death has been reported in previously healthy young adults and children.
The percentage of patients requiring hospitalization appears higher then that would be expected from typical influenza.
The age distribution of hospitalized patients is different than that from seasonal influenza which typically patients are hospitalized are aged less than two years, adults aged greater than 65 years of age and persons with chronic health conditions.
The percentage of patients requiring hospitalization is high among persons aged 30 to 44 years.
Children were twice as susceptible to infection from a household member as adults 19 to 50 years of age and adults older than 50 years of age are less susceptible than younger adults, and the transmissibility of the 2009 H1N1 influenza virus in households is lower than that seen in previous pandemics(Cauchemez S).
Pregnant women appear to have an increased risk of severe disease with the 2009 pandemic influenza A H1N1.
Hospitalization rate for pregnant women was about four times the rate in the general population (Jamieson DJ).
95% pregnant or postpartum patients hospitalized with H1N1 influenza in 2009 were infected in the second or third trimester,and almost 1/5 required intensive care (Louie JK).
Approximately 5% of deaths from the 2009 H1N1 influenza epidemic were associated with pregnancy, even though pregnant women make up only about 1% of the population.
Infants born to women who had been ill with influenza complications suffered with an increased risk of adverse outcomes including preterm birth and small size for gestational age.
Women who were pregnant and were hospitalized with this flu who received a neuraminidase the first 2 days after the onset of symptoms are about one fifth as likely to die as women treated later or not at all.
One third pregnant women with influenza had additional medical conditions besides the pregnancy that were risk factors for influenza complications (Louie JK). Evaluation of 272 patients hospitalized in spring of 2009 for at least 24 hours for influenza like illness and had H1N1 positive testing with PCR: 25% admitted to ICU and 7% died, 45% were under the age of 18 years, 5% were 65 years of age or older and 73% had comorbid problems and few of elderly patients experienced severe illness(Jain S).
Recommendations for H1N1 vaccine is for two doses for young children, although a single 15 mcg doseinactivated vaccine elicits increased influenza specific antibody in more than 90% of healthy infants and young children (Nolan T).
Exposure to in adjuvant influenza A (H1N1) pdm09 vaccine during pregnancy was not associated with a significant increase risk of major birth defects, preterm birth or fetal growth restriction (Pasternak B et al).
Observation studies show that starting treatment even after 48 hours has clinical benefit in influenza.
Receiving influenza vaccine reduces the risk of influenza for pregnant woman and for her infant during the first six months of life.