Millions of Americans are suffering from the influenza virus in what public health experts say is an unusually active and dangerous flu season, the result of several distinct strains of a disease that are likely to kill tens of thousands in the United States.
Influenza activity is widespread in 49 states – all but Hawaii – and flu rates are at high levels in 40 states and in Puerto Rico, according to the Centers for Disease Control and Prevention (CDC). About 6.6 percent of all hospital and doctor visits so far this year are for flu-like illnesses.
Those rates are comparable to the last two notably bad flu seasons – 2014-2015, when an estimated 34 million Americans contracted the flu and 56,000 died, and the 2009 pandemic that sent millions of Americans to the hospital.
“For the past three weeks, the entire country has been experiencing lots of flu all at the same time,” said Dan Jernigan, director of the CDC’s National Center for Immunization and Respiratory Diseases’ Influenza Division.
The influenza virus is among the public health community’s greatest concerns. It is easily communicable, infecting tens if not hundreds of millions of people across the globe every year. And the right strain could also kill a significant number of those infected.
“When we think about a microbe and how bad it is, we think about two things: How easily it spreads, and how nasty it is, its virulence,” said Tom Frieden, a former CDC director. “Flu is that rare bird that potentially could be both easily spreadable and have a relatively high lethality.”
“Flu is the Rodney Dangerfield of diseases,” Frieden added. “People think of it as just the common cold, but it’s not. It knocks you flat.”
Public health experts say this year’s flu season has gotten worse in recent weeks, in most parts of the country. The season is especially bad because three different strains of the virus are being transmitted.
Flu viruses are named for the types of proteins on the surfaces of virus particles; there are 18 different hemagglutinin (H) proteins, and 11 different neuraminidase (N) proteins. This year, the most common strain going around is H3N2, a strain known to cause nastier than usual symptoms.
Another strain is the H1N1 virus, which caused the 2009 pandemic that killed an estimated 150,000 people around the globe.
A third strain is a form of the Influenza B virus, which mainly affects younger people.
“In seasons where H3N2 is the main cause of influenza, we see more cases, more visits to the doctor, more hospitalizations, and more deaths, especially among older people,” Jernigan said. “It’s amazing how much it’s able to evade the human immune system still.”
H3N2 is particularly difficult to stamp out because it is less susceptible to a vaccine than other strains, CDC officials said. The last time H3N2 caused so much illness, in the 2014-2015 season, the vaccine was effective in just 19 percent of those who got a flu shot. About 700,000 Americans were hospitalized with flu-like illnesses that year.
Drug manufacturers must decide well in advance – usually the previous February – what types of flu they will target in the following year’s vaccine. Between that decision and the onset of flu season, the virus might evolve, making the vaccine less effective. Experts say that is likely what has happened this year.
“The strains which are circulating right now are not completely matched with respect to what is in the vaccine,” said Peter Palese, chair of the Department of Microbiology at the Icahn School of Medicine at Mount Sinai in New York City. “There’s an inherent problem that we have to decide in February basically what kind of vaccine will be produced. These variants drift away sometimes.”
This year’s vaccine targets both H3N2 and H1N1, as well as two strains of the Influenza B virus. Jernigan said the CDC is aware that some antiviral drug manufacturers have reported production delays in recent weeks.
Each strain is a threat to different populations. Most older people have gotten some form of the Influenza B virus at some point in their lives, which means they are now immune, and many in the Baby Boom generation are immune from H1N1.
Younger people are more likely to be vulnerable to the B strains and to H1N1. But older populations are more susceptible to H3N2, which first showed up in 1968.
So far this year, 37 children have died of the flu, fewer than the 148 who died during the 2014-2015 flu season – though there are still months to go, and Jernigan said he expected this year’s number to climb.
The CDC does not count adult flu deaths in real time, though it will estimate a final tally later this year.
The H3N2 strain of the flu kills fewer than one in a thousand people who contract it, though CDC officials say children, pregnant women and those over 65 who also have heart conditions are most susceptible. The most deadly form of flu in recent history, the Spanish flu that broke out in 1918-1919, killed 2 to 3 percent of its victims – an estimated 50 million people across the world.
Frieden, the former CDC director, said he is concerned about a new strain of flu, H7N9, that is currently spreading through China. It is more lethal than either the H3N2 strain or the H1N1 strain, but it does not spread as easily among humans – yet.
“The reason we worry about flu is that with just a couple of mutations it could present a risk of a global pandemic that could kill millions of people in a matter of months, before we could even develop a vaccine,” Frieden said.
Palese said there is some hope in the data that suggests this year’s flu season may not be as bad as previous years. The infection curve, which shows how many cases have been reported over a given time period, is beginning to slow, he said.
“I think there is some indication that there is already less of an increase this week compared to two weeks ago,” Palese said. “We should not overhype it up. This is still a regular flu season.”