Across the United States, flu activity remains high in several regions, but is beginning to decline in many areas. However, the ongoing circulation of various respiratory viruses, including COVID-19 and respiratory syncytial virus (RSV), is contributing to a "perfect storm for co-infections."

Where the flu season currently stands

For the week ending Jan. 7, New Mexico was the only state with "very high" levels of influenza-like illnesses (ILI) activity, and 19 states, along with the District of Columbia, reported "high" ILI-activity. Among the other states, 14 reported "moderate" levels of ILI activity, nine reported "low" levels, and seven reported "minimal" levels.

In total, 12,409 patients were hospitalized with influenza for the week ending Jan. 7—a decline from the 18,954 patients who were hospitalized the week before. Currently, the overall cumulative hospitalization rate is 54.4 per 100,000 people—1.8 times higher than the previous high in week one of the 2010-2011 flu season.

So far, CDC has reported 79 pediatric flu deaths during the 2022-2023 flu season, with five deaths reported for the week ending Jan. 7.

Overall, CDC estimates that there have been at least 24 million illnesses, 11 million medical visits, 260,000 hospitalizations, and 16,000 deaths from influenza so far this season.

A 'perfect storm' for multiple respiratory infections

Although flu cases appear to be declining, respiratory viruses, including COVID-19 and RSV, continue to circulate throughout the United States. According to Tina Tan, a pediatric infectious disease specialist at Northwestern University, these three viruses have creased a "perfect storm for co-infections."

Although co-infections can occur at any age, health experts note that children, especially very young ones, appear to be at a higher risk of co-infections than older individuals.

"We've had kids that have actually had three different viruses," Tan said. "Some of them come in with RSV. They've also had influenza and enterovirus. There have been other kids who have presented with COVID and influenza."

Shikha Garg, a CDC medical epidemiologist, noted that around "20% of infants less than six months of age hospitalized with the flu have had co-infection with RSV." As children grow older, this number declines to roughly 13% of children ages six months to two years and less than 5% for children ages five and older.

There is also growing evidence that co-infections can result in more severe illness than an infection with a sole virus. For example, a new CDC study that examined more than 4,000 hospitalized children found that those who had COVID-19 and another virus were significantly more likely to require oxygen and need intensive care.

"We found that children under five had about twice the odds of having severe illness when they had a [co-infection] compared to when they just [a] SARS-CoV2 infection," said Nickolas Agathis, a pediatrician and CDC medical officer who led the study. "The children under two who had RSV were twice as likely to have severe illness compared to children who just had COVID and not RSV also."

According to NPR, multiple infections at the same time may lead to increased inflammation, and different respiratory viruses may damage the lungs in different ways, leading to more severe symptoms overall.

"It's almost as though you're getting punched more than once, and that can make you sicker," said William Schaffner, an infectious disease specialist at Vanderbilt University.

To reduce the risk of infection from any virus this winter, health experts recommend a few general tips:

• Get vaccinated against both COVID-19 and the flu
• Wash your hands regularly
• Wear a mask in crowded areas with poor ventilation and around those who are sick

"Getting vaccinated with influenza and SARS-CoV2 vaccines and staying up to date on that is critical for protecting children as well as community and family members," said Fiona Havers, a medical epidemiologist at the CDC. "Staying home when you're sick is critical." (Carbajal, Becker's Hospital Review, 1/13; Stein, "Shots," NPR, 1/18)