Could seasonal flu help pandemic flu?

pillsNew Zealand-based research has shown that, for people who get both seasonal A(H1N1) and pandemic flu (H1N1) flu at the same time, the results are more than just feeling very ill – the combination could lead to a newer, more harmful variant of the virus which is harder to treat.

The new strain could arise if a cell is infected by both flu viruses (co-infection), one of which is drug-resistant (DR). This ability could be passed on to the non-DR virus, which, if the viruses are seasonal and pandemic flu, could lead to a drug resistant pandemic flu virus.  Of particular concern is the possibility of a pandemic flu virus that is resistant to oseltamivir (Tamiflu).

Scientists, including researchers from ESR, looked at 11 New Zealanders who were infected in this way in 2009, and have concluded that, while co-infections are rare, the scenario is possible.  The paper is published in the CDC’s journal Emerging Infectious Diseases.

The SMC approached local experts to comment on the research and its significance. The paper can be accessed in the SMC Resource Library.


Dr Matthew Peacey (lead author of the paper), of ESR, comments:

“Last year our influenza season began with the circulation of the regular seasonal A/H1N1 strain which is Tamiflu resistant. This seasonal A/H1N1 strain was rapidly overtaken by the worldwide pandemic A/H1N1 strain.  In New Zealand there was a short 4-5 week period when both strains were circulating within the community and in some cases both strains were able to infect a single patient. When this happens influenza viruses are able to swap genes.  In fact, this natural swapping of genes was how the pandemic strain came about.

“The worry here is this event could give rise to a more virulent influenza strain, such as a pandemic strain that is resistant to Tamiflu, and so needs to be monitored closely.”

Professor John Fraser, Head of School of Medical Sciences, University of Auckland, comments:

“This is a potentially significant discovery confirming previous reports that suggested humans can be simultaneously infected by two different strains of influenza A.

“This finding immediately begs the question of whether humans can act a reservoir for viral resortment or gene mixing to generate more virulent strains. If the person is taken anti-viral medication such as Tamiflu, this could select for a resorted drug resistant strain.

“Whether resortment actually occurs during human co-infection and if so at what rate, remains a key question.”

Dr Anne La Flamme, Senior Lecturer in the School of Biological Sciences at VUW, comments:

“The recent study by Peacey et al takes an interesting step forward and shows that during the 2009 influenza pandemic in New Zealand, there were patients harboring both the seasonal and pandemic H1N1 strains. Co-infections with different influenza strains are thought to be rare and this report shows that of the samples tested only 1.1% were positive for both the seasonal and pandemic H1N1 strains. It is important to note that they did not look for co-infections with any other influenza A or B strains.

“The findings that make this study interesting are 1) these cases did not appear to be linked (they were scattered all over New Zealand) and 2) all of the co-infections were from samples taken between 14-30 June (when both strains were equally dominant in the community). These findings suggest that co-infection can be sporatic but occurs when you have co-dominant strains.

“Overall, the importance of this study is in highlighting that the viruses have the opportunity for reassortment (i.e. exchanging gene segments) in patients who are co-infected with 2 different strains and provides insight on the conditions that may encourage this opportunity. The threat is that this reassortment could change a previously drug-sensitive strain to a drug-resistant one. This event did not happen during the 2009 season, but the opportunity for it to occur did.”


For more information, or to speak to a New Zealand expert on this topic, contact the Science Media Centre (NZ) on tel: 04 499 5476 or email: