Influenza A H1N1 infections have high transmissibility, a short incubation period, and high rates of morbidity and mortality, according to a recent study.

During the 2009 H1N1 pandemic, 59 million people were infected, resulting in 265,000 hospitalizations and 12,000 deaths in the United States, according to the World Health Organization.

Research published in BMC Infectious Diseases described in patients with suspected and confirmed Influenza A H1N1 infection who were prescribed oseltamivir, the risk factors associated with infection, the association between infection and mortality, and the factors associated with in-hospital mortality in infected patients.

​White and yellow antiviral pills. Source: Getty

The study, conducted in a tertiary care, private hospital in São Paulo, Brazil, with retrospective review from January 2009 to December 2015, focused on inpatients with suspected and confirmed Influenza A H1N1 infection who were prescribed oseltamivir, the risk factors associated with infection, the association between infection and mortality, and the factors associated with in-hospital mortality in infected patients.

Identified for the study were 1,405 inpatients who underwent polymerase chain reaction (PCR testing and received treatment with oseltamivir. The study also pinpointed 157 patients with confirmed Influenza A H1N1, a third of whom were diagnosed in the pandemic period.

The study noted that antiviral therapy was prescribed according to the institutional protocol], with oseltamivir initiated empirically based on clinical presentation or after a positive PCR test. Symptoms, such as fever, cough, sore throat, runny and/or stuffy nose, muscle or body aches, headaches, and fatigue was used to determine whether to administer antiviral therapy, which was mandated in patients at high risk for developing influenza- related complications.

Brazilian researchers reported no difference in mortality between cases, who underwent PCR testing and received treatment with oseltamivir vs. controls. “We observed a high proportion of negative PCRs among patients treated with oseltamivir because the treatment was administered empirically based on symptoms,” the study team emphasized. “The recommendation is to initiate the treatment with oseltamivir within 48 h of Influenza symptom onset, especially in critically ill patients, in order to reduce symptom duration, complications such as pneumonia, and possibly death However, empiric therapy leads to uninfected patients receiving treatment and testing modalities other than PCR often have low sensitivity and specificity.”

Several factors made a difference in mortality, however. Researchers noted that immunocompromised status, requirement of vasoactive drugs, mechanical ventilation, acute hemodialysis, albumin administration, surgical procedures and thoracic procedures and length of stay were associated with increased risk of death in Influenza A H1N1 infected patients.

“In conclusion, the profiles of the infected and uninfected patients were very similar and there was no difference in mortality,” study authors wrote. “The only risk factor associated with death in infected patients was an immunocompromised state."