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By Shamard Charles, M.D.

The World Health Organization issued new recommendations Thursday on the composition of the influenza vaccines for use in the 2019-20 flu season in the Northern Hemisphere.

A WHO advisory group issued the recommendations after a meeting in Beijing sponsored by the National Institute for Viral Disease Control and Prevention, which is part of the Chinese Center for Disease Control and Prevention, the country's leading public health agency.

Between September and January, the H1N1, H3N2 and influenza B strains predominated globally. Most cases this season are the milder H1N1 variety.

The advisory board recommended that next year’s flu vaccine cover the H1N1 strain and two types of the influenza B virus. The H3N2 strain will also be covered, the board said. The specific subtype will be announced in March.

“In light of recent changes in the proportions of genetically and antigenically diverse A(H3N2) viruses, the recommendation for the A(H3N2) component has been postponed,” WHO said in a statement.

WHO convenes advisers in February and September each year to recommend viruses for inclusion in the vaccines for the upcoming influenza season. Flu strains are different in the Northern and Southern hemispheres, so WHO holds separate meetings to discuss each region.

The recommendations are used by the national vaccine regulatory agencies and pharmaceutical companies to develop, produce and license influenza vaccines.

Every flu vaccine is a cocktail, aimed at either three or four of the most common flu strains. Flu vaccines must be reformulated every year because flu viruses mutate constantly in a process called antigenic drift. This season’s flu shot contains four strains — H1N1, H3N2, Influenza A and Influenza B — in what’s known as a quadrivalent vaccine.

So far this year’s flu season has been mild, in part due to a good match between the virus strains in circulation and this season’s flu vaccine.

In its weekly report last week, the CDC said the interim estimate of the flu vaccine’s effectiveness against the H1N1 influenza strain was 47 percent. For context, at this time last year the midseason effectiveness of the vaccine was just 36 percent. Sixty percent effectiveness for the entire season is considered a great number.

“One of the main reasons we give flu vaccine is to prevent complications," said Dr. William Schaffner, an infectious-disease expert at Vanderbilt University and medical director of the National Foundation for Infectious Diseases. "Milder illness means people are less likely to have the complications that land them in the hospital.”

"A flu vaccine that matches well with the viruses that are circulating prevents complications, severe illness and deaths,” Schaffner told NBC News.

This season, nearly 90 percent of the flu cases tested by the CDC are turning out to be H1N1, the milder variety. A milder dominant strain and a more effective vaccine have resulted in less hospitalizations and deaths this season. Last season, 50 million people caught flu and 80,000 died in the worst flu season in decades. Currently, the CDC estimates 11,600 to 19,100 deaths from Oct. 1 to Feb. 9 in what it has called “a low-severity influenza season.”