Two research groups today reported rare cases of pandemic 2009 H1N1 coinfections with seasonal influenza strains, including a cluster in Cambodia and two patients from Italy´s Veneto region.

Dual infections involving the 2009 H1N1 virus and seasonal strains have been reported before and are to be expected, a spokesman for the US Centers for Disease Control and Prevention (CDC) said today. Though they are rare, they bear watching due to the possible reassortment between influenza A strains.

The coinfections in Cambodia involved influenza A (H3N2), and the patients in Italy had the 2009 H1N1 virus along with influenza B.

The Cambodian coinfections occurred in a community in central Cambodia during the fall of 2009, according to a report published today by US and Cambodian researchers in the American Journal of Tropical Medicine and Hygiene.

Shortly after a 23-year-old man was diagnosed in a local clinic as having a 2009 H1N1 infection, three boys who lived in the man´s household were seen at the clinic for flulike symptoms. A day after they got sick, their teacher, a 24-year-old woman, also became ill, and health providers obtained a respiratory sample from her.

Specimens revealed that one of the boys and the teacher were coinfected with the 2009 H1N1 virus and the seasonal H3N2 strain. Virus isolation confirmed the 2009 H1N1 infection in the man and the coinfections in one of the boys and his teacher. Tests suggested the other two boys had H3N2 infections.

Genetic analysis found no evidence of recombination, according to the report. The H3N2 isolate was 99% similar to the Perth H3N2 component of the 2010-11 vaccine strain, and the 2009 H1N1 isolate showed 99.3% similarity to a reference strain.

The researchers noted patients infected with the two strains did not have more severe disease than other flu patients and did not require hospitalization. They added that none of the patients had received the pandemic or seasonal flu vaccine or were treated with neuraminidase inhibitors.

They concluded that transmission of the 2009 H1N1 virus when seasonal flu viruses are circulating can raise the possibility of reassortment, and that national and international cooperation is needed to watch for the emergence of novel or reassorted flu viruses.

In the second report, Italian researchers describe two patients who were coinfected with the 2009 H1N1 virus and influenza B. Their report was published today in Virology Journal. The infections were identified during the testing of 1,403 hospitalized patients for influenza from October 2010 to April 2011.

One of the patients was a 54-year-old kidney transplant recipient, and the other was a 6-month-old baby who had severe respiratory symptoms and pneumonia.  Both infections were confirmed by real-time polymerase chain reaction testing and viral isolation.

Neither patient had been vaccinated. The transplant patient, who had mild symptoms, was treated with oseltamivir (Tamiflu), due to his or her immunocompromised status. The baby was hospitalized and received intravenous and nebulized antibiotics, but not antiviral medication. The child recovered quickly and was discharged from the hospital about a week after starting antibiotic treatment. The kidney transplant patient fully recovered.

The researchers noted that both patients had weakened immune systems, which might have played a role in their susceptibility to the coinfections.

CDC spokesman Tom Skinner told CIDRAP News that the coinfection reports are not surprising. "2009 H1N1 is now a human seasonal virus, and it´s certainly possible for people to be coinfected with more than one influenza virus at the same time," he said.

Coinfections with two influenza A viruses can increase the opportunity for genetic swapping between viruses and emergence of influenza A viruses that have the properties of the 2009 H1N1 strain, he said.

In September 2010, researchers who tested more than 1,000 2009 H1N1 cultures for antiviral resistance found 11 cases of coinfections with oseltamivir-resistant seasonal H1N1. All of the samples were collected before the 2009 H1N1 vaccine was available.

In May 2010, Hong Kong researchers reported on a renal transplant patient who was infected with both the 2009 H1N1 virus and the H3N2 strain. Researchers have also reported dual infections in a college campus outbreak near Beijing and have identified seasonal H1N1 coinfections during screening in New Zealand.

Myers CA, Kasper MR, Yasuda CY, et al. Dual infection of novel influenza A/H3N2 in a cluster of Cambodian patients. Am J Trop Med Hyg 2011 Nov;85(5):961-3 [Abstract]

Calistri A, Salata C, Cosentino M, et al. Report of two cases of influenza virus A/H1N1v and B coinfection during the 2010/2011 epidemics in the Italian Veneto region. Virol J 2011 Nov 3;8(502) [Abstract]

See also:

Sep 15, 2010, CIDRAP News Scan

May 20, 2010, Ann Intern Med letter extract

May 15, 2010, Clin Infect Dis abstract

October 2010 Emerg Infect Dis abstract