we requested that all us state public health laboratories submit influenza - positive specimens for virologic surveillance , including antiviral susceptibility testing , as described ( 2 ) . in brief , every 2 weeks each laboratory was asked to send 5 specimens for all virus types for virus isolation and neuraminidase ( na ) inhibition assay for oseltamivir , zanamivir , and , in a subset , laninamivir and peramivir ( 3 ) . all oseltamivir - resistant viruses were tested for the h275y substitution in na by pyrosequencing ( 4 ) . unpropagated influenza a(h1n1)pdm09 virus  positive clinical specimens were screened for the h275y substitution by pyrosequencing ( technical appendix ) . if a cluster ( > 2 viruses ) of oseltamivir - resistant a(h1n1)pdm09 viruses was detected , the state was asked to submit additional influenza a(h1n1)pdm09 virus specimens for testing . we attempted to collect information by using a standard form from all patients with oseltamivir - resistant virus infection and from a sample of patients with oseltamivir - susceptible virus infection . a 2:1 ( susceptible : resistant ) sample was randomly selected from the list of tested specimens from the same age group in each state ( < 5 , 517 , 1864 , and 65 years ) . patients with oseltamivir - resistant or -susceptible virus infections were compared by using conditional logistic regression models that controlled for age . full na and hemagglutinin sequence analysis was performed on all resistant viruses and a subset of susceptible viruses . during october 1 , 2013april 30 , 2014 , a total of 4,968 influenza a(h1n1)pdm09 virus specimens collected from 50 us states and 2 territories were tested for antiviral susceptibility ( 1,811 virus isolates and 3,157 clinical specimens ) . a total of 59 ( 1.2% ) influenza a(h1n1)pdm09 viruses from 20 states had the h275y na substitution conferring resistance to oseltamivir and peramivir ( figure 1 ; table 1 ) . number of oseltamivir - resistant a(h1n1pdm)09 viruses divided by total number of viruses tested is shown for each state . oseltamivir - resistant a(h1n1)pdm09 viruses were significantly more prevalent in louisiana ( la ) ( p = 0.04 , by fischer 2-sided exact test ) , pennsylvania ( pa ) ( p<0.001 ) , mississippi ( ms ) ( p = 0.02 ) , hawaii ( hi ) ( p = 0.02 ) , and california ( ca ) ( p = 0.03 ) than in all other states combined . * most ( 99.8% ) influenza a(h1n1)pdm09 virus isolates were characterized as a / california/7/2009-like , the influenza a ( h1n1 ) component of the 20132014 northern hemisphere influenza vaccine . includes pyrosequencing data from new york contract laboratory and data submitted by 19 state public health laboratories in arizona , california , colorado , delaware , florida , georgia , hawaii , idaho , maine , maryland , massachusetts , michigan , minnesota , new york , pennsylvania , texas , utah , washington , and wisconsin . viruses with the h275y substitution were detected in patient specimens collected during october 7 , 2013march 25 , 2014 ; monthly prevalence ranged from 0.8% to 2.5% . among 49 ( 83.0% ) patients with a resistant virus infection and available information , 15 ( 30.6% ) received oseltamivir before specimen collection ( table 2 ) . prior oseltamivir use was more frequent among hospitalized patients and patients with resistant virus infections than those with susceptible virus infections . among those with prior exposure , 6 ( 40.0% ) patients with oseltamivir - resistant and none with oseltamivir - susceptible virus infections were immunocompromised ( p = 0.03 ) . no differences were found between patients with oseltamivir - resistant or -susceptible virus infections . * or , odds ratio adjusted for age group ; iqr , interquartile range ; na , not applicable . long history of steroids treatment , hiv / aids , solid organ transplant , lupus , solid tumor malignancy , hypothyroidism , leukemia , and pituitary condition .   morbid obesity , chronic liver disease , neurologic disorders , chronic kidney disease , seizure , epilepsy , and depression . most resistant viruses were clustered in 5 states ( california , hawaii , louisiana , mississippi , and pennsylvania ) ( figure 1 ) . among patients with oseltamivir - resistant virus infection , only 1/4 from california , 0/4 from hawaii , 3/11 from louisiana , 1/3 from mississippi , and 0/14 from pennsylvania had exposure to oseltamivir before specimen collection . all patients from pennsylvania except 1 attended 1 of 2 universities ( among 7 participating students , none shared classes , residences , or social events ) . limited information was available for oseltamivir - treated patients with resistant and susceptible virus infections ( technical appendix table ) . most hemagglutinin sequences from us influenza a(h1n1)pdm09 viruses collected since october 1 , 2013 , belonged to the 6b genetic group , and there was minimal separate clustering between susceptible and resistant viruses ( technical appendix 
 figure 1 ) . similar results were observed for the phylogenetic tree of the na gene ( figure 2 ) . na sequences from resistant viruses in the united states with the h275y substitution were generally scattered among other susceptible viruses from genetic group 6b viruses . most ( > 99% ) influenza a(h1n1)pdm09 viruses currently in circulation have na substitutions v241i and n369k ( technical appendix 
 figure 2 ) . there was > 1 cluster of na sequences with the n386k substitution ; each cluster contained susceptible and resistant viruses . most ( > 89% ) resistant viruses from the united states do not have the n386k mutation . evolutionary relationships among influenza a ( h1n1)pdm09 virus neuraminidase genes , united states , 201314 . phylogenetic tree was generated by using mega software v5.2 ( http://www.megasoftware.net/ ) and the neighbor - joining method . a / california/07/2009 ( current northern hemisphere vaccine strain ) virus was used as a reference for ancestry ( root ) and numbering . f , centers for disease control and prevention reference antigen ; oct , october 2013 ; nov , november 2013 ; dec , december 2013 ; jan , january 2014 ; feb , february 2014 ; gly , glycosylation . during the 201314 influenza season , prevalence of oseltamivir - resistant influenza a(h1n1)pdm09 viruses was low ( 1% ) in the united states , although prevalence was higher in a few states . most patients infected with an oseltamivir - resistant influenza a(h1n1)pdm09 virus had no prior exposure to oseltamivir . these findings are consistent with a low , and locally variable , level of circulation of resistant viruses . in our study , exposure to oseltamivir before specimen collection was more common among hospitalized patients with resistant virus infections than those with susceptible virus infections . we can not differentiate whether these viruses emerged during treatment or were present before treatment , but many patients were immunocompromised , a condition associated with emergence of resistance during treatment ( 5 ) . before 2007 , resistance to na inhibitors among influenza viruses circulating globally was low ( < 1% ) ( 6 ) . however , the 200708 influenza showed an emergence of oseltamivir - resistant seasonal influenza a(h1n1 ) h275y viruses at variable prevalence ( 6 ) , and by the 200809 season , many countries were reporting up to 100% oseltamivir resistance ( 7 ) . the sharp increase in seasonal influenza a(h1n1 ) h275y viruses from < 1% to 100% was not attributed to oseltamivir use ( 8) , but was probably caused by evolutionary advantage of h275y variants . studies suggest that permissive na mutations , including r222q , v234 m , and d334n , counteracted the detrimental effect of h275y on na function and virus replicative properties , thus enabling virus to remain fully functional ( 9 ) . the exact mechanism(s ) responsible for evolutionary advantage of seasonal influenza a(h1n1 ) h275y viruses over oseltamivir - susceptible viruses remain unknown . since emergence of influenza a(h1n1)pdm09 virus in 2009 , there is concern that the h275y substitution may become fixed in the viral genome , as it did in seasonal influenza a(h1n1 ) virus in 200809 . oseltamivir - resistance among influenza a(h1n1)pdm09 viruses during their first 2 seasons in circulation ( 200911 ) remained low ( < 1% ) ( 2,5 ) . august 2011 , in newcastle , new south wales , australia , a cluster of oseltamivir - resistant influenza a(h1n1)pdm09 h275y viruses was detected among patients without prior oseltamivir exposure ( 10 ) , suggesting community transmission . these h275y viruses had permissive mutations , v241i and n369k , in addition to n386s ( 10 ) , which was similar to h275y viruses isolated in 2012 from dutch travelers returning from spain ( 11 ) . these mutations were believed to offset the destabilizing effect of h275y and possibly enhance virus transmissibility . the substitutions v241i , n369k , or n386s were not present in influenza a(h1n1)pdm09 virus when it emerged in 2009 . however , since 2011 , circulating influenza a(h1n1)pdm09 viruses have acquired these substitutions , coinciding with increasing evidence for community transmission of influenza a(h1n1)pdm09 h275y viruses in the united states and other countries ( 2,12 ) . all influenza a(h1n1)pdm09 viruses circulating in the united states in 201314 had v241i and n369k substitutions , and 10% of resistant viruses and 20% of susceptible viruses had an additional substitution ( n386k ) . all influenza a(h1n1)pdm09 h275y viruses detected in china and japan in 201314 had all 3 substitutions ( 13 ) . in combination with the h275y substitution , v241i or n369k enhances surface expression and activity of na ( 14 ) . the n386s substitution and the recently detected n386k substitution result in loss of a glycosylation site ( 15 ) . although the potential role of these changes in virus spread was suggested ( 10 ) , no direct evidence is available . close monitoring for the n386k / s substitution may provide information needed to delineate its role in virus spread . in addition to permissive na mutations , other properties , such as antigenic novelty , which might provide an advantage to oseltamivir - resistant viruses and facilitate their spread , should also be monitored . the potential for emergence and spread of oseltamivir - resistant influenza a(h1n1)pdm09 viruses , coupled with limited pharmaceutical options against influenza , emphasizes the need for local surveillance for na inhibitor susceptibility among circulating influenza viruses . studies on biologic characteristics ( e.g. , replication in and transmissibility from ferrets ) of influenza a(h1n1)pdm09 virus community isolates with h275y and other permissive mutations are ongoing . supplementary methods for analysis of oseltamivir - resistant influenza a(h1n1)pdm09 viruses , united states , 201314 .
we report characteristics of oseltamivir - resistant influenza a(h1n1)pdm09 viruses and patients infected with these viruses in the united states . during 201314 , fifty - nine ( 1.2% ) of 4,968 
 analyzed us influenza a(h1n1)pdm09 viruses had the h275y oseltamivir resistance  conferring neuraminidase substitution . 
 our results emphasize the need for local surveillance for neuraminidase inhibitor susceptibility among circulating influenza viruses .