The United States is currently navigating a severe influenza season characterized by a dominant H3N2 strain, including a new subclade K variant, and increasing hospitalization rates across all age groups. Early indicators from the southern hemisphere suggested a potentially challenging season, a trend now observed in rising U.S. flu cases and associated deaths. While many health officials and experts continue to recommend vaccination, official messaging regarding vaccine efficacy and public health campaigns has undergone changes, contributing to concerns about decreased vaccine uptake.
Overview of the 2023-2024 Flu Season
Early data from the southern hemisphere, which experienced notable and longer-duration flu activity, suggested a potentially challenging influenza season for the United States, according to Richard Webby, a flu researcher at St. Jude Children's Research Hospital. The Centers for Disease Control and Prevention (CDC) has identified the current flu season as severe, with health officials noting a record number of illnesses and strained hospital resources.
As of January 9, CDC estimates reported approximately 15 million illnesses, 180,000 hospitalizations, and 7,400 deaths, including at least 17 children, in the U.S. By December 20, an estimated 7.5 million influenza cases and 81,000 hospitalizations had been reported, representing a 25% increase in cases compared to the preceding week. At that time, 32 states reported high or very high levels of influenza illness. Jennifer Nuzzo, an epidemiologist at Brown University, stated that a concentrated surge in cases presents greater challenges than cases spread over a longer duration, potentially straining healthcare capacity.
Colorado is experiencing a significant increase in flu hospitalizations, with a notable rise among children, according to Dr. Shen Nagel, a pediatrician at Pediatrics West. The state also recorded its first pediatric flu death of the season. Flu-like respiratory illnesses accounted for about 8.2% of doctor visits as of early January, marking the highest rate for this period since the 1997-98 season. This rate decreased to 7.2% the following week, a drop the CDC suggested could be due to holiday-related delays in reporting or seeking care.
The previous flu season recorded 279 influenza-associated pediatric deaths, which was the highest number reported to the agency during a seasonal influenza epidemic outside of a pandemic. Mortality among children is increasing, with nearly 300 pediatric flu deaths last season.
Dominant Strains and Vaccine Match
The CDC flu tracker identifies H3N2 as the dominant flu virus strain in the northern hemisphere. H3N2 viruses are generally associated with higher activity and a greater incidence of severe disease, as observed during the 2016-2017 flu season. A new variant of the H3N2 virus, specifically subclade K, has emerged and become the prevalent strain in the U.S.
Epidemiologist Caitlin Rivers from Johns Hopkins explained that this mutation may enable the virus to circumvent existing immunity from previous infections or vaccinations. Scientists predict dominant flu strains months in advance to develop the annual vaccine. This year, the circulating newer strain has resulted in a less optimal match for the current flu vaccine. Jennifer Nuzzo indicated that while protection against the new variant might be reduced, vaccination is still expected to offer significant benefits.
Vaccine Efficacy and Uptake
Data from the U.K. suggest that vaccines provide protection against severe outcomes, indicating effectiveness of approximately 70% to 75% against hospitalization in children and 30% to 40% against hospitalization in adults. Early data from the southern hemisphere indicates that vaccinated children may have a 50% to 75% lower likelihood of hospitalization due to flu this season. Vaccination typically reduces illness severity, potentially leading to shorter illness duration and contagious periods. Dr. Seema Lakdawala, an associate professor of microbiology and immunology at Emory School of Medicine, stated that the vaccine remains effective at preventing severe illness, even if it does not fully stop transmission.
Despite expert consensus on the vaccine's benefits, current vaccination rates are 42.5% for children and 43.5% for adults, a decrease from the 2019-20 season's peak of 63.7% for children and 48.4% for adults. Dr. Megan Berman, a professor of internal medicine, observed a rise in hospitalized patients who had not been vaccinated. Caitlin Rivers expressed concerns regarding potentially lower uptake of both influenza and COVID-19 vaccines this year. Dr. Nagel suggested potential contributing factors to lower flu vaccination rates included changes at higher government levels, such as the CDC and HHS, and subsequent communication.
Official Statements and Policy Adjustments
In early January, flu vaccine recommendations for children transitioned to a "shared clinical decision-making" model. Robert F Kennedy Jr, Secretary of the US Department of Health and Human Services, suggested that fewer children getting vaccinated might be "a better thing," stating a lack of scientific evidence for the flu vaccine's efficacy in preventing severe illness, hospitalizations, or death in children. This statement was made despite existing studies, including those from the CDC, indicating the vaccine's effectiveness. Mehmet Oz, Administrator for the Centers for Medicare & Medicaid Services, also questioned the effectiveness of flu vaccines, advocating for self-care.
The CDC's public statements have characterized the decision to vaccinate as a "personal one," advising individuals to consult their doctors regarding "potential risks and benefits." Differing from previous years, the CDC has not issued health advisories for the new variant, top health officials have not publicly received vaccinations, and a "Wild to Mild" campaign was discontinued. Epidemiologists have expressed concerns regarding potential decreases in flu vaccine uptake, attributed in part to the CDC's decision, under Secretary Kennedy Jr., to discontinue its public vaccination campaign.
As of early December, approximately 130 million flu vaccines had been distributed in the U.S., a decrease of 13 million compared to the same period in the previous year. Nuzzo commented on the absence of a comprehensive vaccination encouragement plan despite the previous season's pediatric flu mortality rates and current viral circulation patterns. A spokesperson for the Department of Health and Human Services (HHS) stated in an earlier period that it was "too early to know what viruses will spread this season, in what proportion, and how well the vaccine will work in the United States."
Antiviral Treatments and Prevention Measures
Antiviral treatment usage among hospitalized flu patients has declined, from 90% in 2018-19 to 79% in 2022-23. This reduction was more pronounced in children, with three out of five hospitalized children receiving antivirals in 2022-23, compared to four out of five in 2018-19. Only 28% of children under five seeing a doctor for flu and 32% of children in emergency departments receive these treatments.
Nuzzo highlighted the availability of at-home flu tests at pharmacies and the importance of early administration of medication for individuals testing positive for influenza. Dr. Berman advised that patients sick enough to seek medical care, as well as those with underlying conditions, pregnant individuals, and older people, should receive antivirals, which are effective against the current variant and should be started promptly after a positive test.
Health officials advise individuals who have not yet received a flu shot to do so promptly, as immunity typically develops within two weeks of vaccination. Flu activity is expected to continue for several months, making current vaccination still beneficial. Preventative measures against illness include frequent hand washing, keeping sick family members home until fevers have been absent for at least 24 hours, avoiding gatherings if someone is actively ill, and vaccination. Dr. Lakdawala emphasized that the COVID-19 pandemic demonstrated the capacity to control the flu disease burden through measures like vaccination, antiviral treatments, mask-wearing, improved ventilation, hand washing, covering coughs and sneezes, and staying home when sick. Some health systems, such as in New York, have implemented mask mandates for providers due to the increase in cases.
Comparison with Other Respiratory Viruses
In recent months, COVID-19 activity in Colorado has been relatively low and generally milder in children compared to the flu. The flu is currently observed to cause more severe illness and longer duration in children, particularly regarding muscle aches, than COVID-19. Recent COVID-19 waves have predominantly affected older adults and high-risk patients, while flu is impacting all age groups, leading to more hospitalizations than COVID-19 currently. The state has not experienced a "tripledemic" this season, unlike 2022 when flu, RSV, and COVID-19 surged simultaneously. RSV activity remains low, with its peak typically occurring in January and February. High rates of COVID-19 and RSV are also contributing to overall hospital strain, with both childhood vaccines for these viruses also facing recent restrictions.
Future Considerations
There is concern regarding the H5N1 bird flu, which continues to affect flocks across the U.S. The potential for two flu variants to combine and create a deadlier variant through a process called reassortment is also a recognized risk.