This past flu season is believed to have been milder than previous years.
Rates of outpatient illness and mortality were generally lower than usual, MedPageToday reported.
The rates of hospital admission were only higher than in past seasons for people between the ages of 50 and 64.
The season was dominated by the pH1N1 flu strain rather than the H3N2 influenza A strains, which may be deadlier.
Researchers looked at 308,741 respiratory samples that were gathered from Sept. 29 through May 17; 53,470 of these samples turned out to be positive. Out of the positive samples 87.4 percent were influenza A and 12.6 percent were influenza B. Of the influenza A viruses 90.3 percent were pH1N1 viruses and 9.7 were influenza A (H3) viruses, MedPage Today reported.
The researchers identified a A (H3N2) virus, which is seen more often in pigs than in humans. Ninety-nine percent of the pH1Ni viruses were similar to the year's vaccine strain. Next year's vaccine strain will be the same according to the Centers for Disease Control and Prevention (CDC), MedPage Today reported.
The data was tracked by the CDC's Outpatient Influenza-Like Illness Surveillance Network.
People between the ages of 18 and 64 made up 57.4 percent of admissions; the incidence rate was about 35.6 admissions per 100,000 people. For people between 50 and 64 was at 54.3 per 100,000 people compared with the 8.1 to 40.6 range seen in recent years.
The CDC said the spike in that age range is "likely attributable to several factors," such as lower vaccination coverage, MedPage Today reported.
"The theory of herd immunity is that if you vaccinate enough people, not everyone has to be immune in order to stop the disease spreading through a community," Dr. Michael Jhung, a medical officer in the U.S. Centers for Disease Control and Prevention's influenza division, told HealthDay.
