What was the 2017–2018 flu season like?
The 2017–2018 influenza season was marked by high severity. It brought a sharp rise in outpatient and emergency department visits for influenza-like illness (ILI), elevated rates of flu-related hospitalizations, and widespread, prolonged activity across the country. In 2017, CDC introduced a new method to measure seasonal severity, applying it retrospectively to the 2003–2004 through 2016–2017 seasons. The 2017–2018 season became the first to be classified as “high severity” across every age group.
When did the 2017–2018 flu season peak?
Flu activity began climbing in November and reached a lengthy stretch of intense activity in January and February, remaining high through late March. ILI peaked at 7.5%—the highest level since the 2009 pandemic (7.7%). Overall, ILI stayed at or above the national baseline for 19 consecutive weeks, making this one of the longest flu seasons in recent memory.
For additional details, see the MMWR: 2017–18 Influenza Activity Update or explore the FluView Interactive dashboard.
How many people died from flu during the 2017–2018 season?
Flu-related deaths in children are reported directly to CDC, but adult flu deaths are not nationally notifiable. To track influenza-related mortality across all age groups, CDC relies on the National Center for Health Statistics (NCHS) Mortality Reporting System, which monitors death certificates that list pneumonia or influenza (P&I) as either an underlying or contributing cause of death. While this system indicates when flu-related deaths are elevated, it does not provide an exact nationwide count.
During the 2017–2018 season, the percentage of deaths linked to pneumonia and influenza stayed at or above the epidemic threshold for 16 consecutive weeks. By comparison, over the previous five seasons, this indicator averaged 11 weeks above the threshold (with a range of 7–15 weeks). At its peak, during the week ending January 20, 2018, P&I mortality reached 10.8% and remained above 10% for four straight weeks.
To estimate the broader burden, CDC uses mathematical modeling, just as it does for cases, medical visits, and hospitalizations. Based on this approach, influenza-associated deaths in the United States have ranged from a low of about 12,000 (in the 2011–2012 season) to a high of around 56,000 (in 2012–2013). For these calculations, death certificate data and weekly influenza surveillance are combined to estimate how many deaths tied to respiratory or circulatory causes can be attributed to flu.
For more information, see Estimating Seasonal Influenza-Associated Deaths in the United States and CDC’s Disease Burden of Influenza page.
How many children died from flu during the 2017–2018 season?
As of April 19, 2019, CDC had received reports of 186 pediatric deaths linked to influenza during the 2017–2018 season. This was the highest number of flu-associated child deaths ever recorded for a regular flu season, surpassing the previous high of 171 reported in 2012–2013. Notably, about 80% of the children who died had not been vaccinated that season. For the most up-to-date information, see FluView: Influenza-Associated Pediatric Mortality.
Since flu-associated pediatric deaths became nationally notifiable in 2004, reported totals have varied widely by season, ranging from 37 deaths in 2011–2012 to 186 in 2017–2018 (as of April 2019). The exception was the 2009 influenza pandemic, during which 358 pediatric deaths were reported between April 2009 and October 2010.
How many people were hospitalized from flu during the 2017–2018 season?
Between October 1, 2017, and April 28, 2018, the Influenza Hospitalization Surveillance Network (FluSurv-NET)—which covers about 9% of the U.S. population—recorded 30,453 laboratory-confirmed flu-related hospitalizations. Adults 65 years and older made up roughly 58% of these cases. Hospitalization rates across all age groups were the highest ever documented in this system, surpassing the record set during the 2014–2015 season, another severe H3N2-predominant year. At that time, CDC estimated that FluSurv-NET hospitalizations represented about 710,000 total hospitalizations nationwide. Final estimates for the 2017–2018 season were expected in fall 2018.
What flu viruses circulated during the 2017–2018 season?
Overall, influenza A(H3N2) viruses dominated during the 2017–2018 season. However, by early March and through May 2018, influenza B viruses became more commonly reported than influenza A viruses.
How much flu vaccine was produced and distributed during the 2017–2018 season?
Because flu vaccines are produced by private manufacturers, supply depends on their output. For the 2017–2018 season, manufacturers projected between 151 million and 166 million doses of injectable vaccine for the U.S. market. By February 23, 2018, approximately 155.3 million doses had been shipped—a record distribution of flu vaccines. More details on vaccine supply are available on CDC’s Seasonal Influenza Vaccine Supply & Distribution page.
How effective was the 2017–2018 flu vaccine?
The overall effectiveness (VE) of the 2017–2018 influenza vaccine against both A and B viruses was estimated at 40%. In other words, vaccination lowered the risk of seeking medical care for flu illness by about 40%. When broken down by type and subtype, effectiveness was estimated at 25% for A(H3N2), 65% for A(H1N1), and 49% for influenza B viruses. These estimates were presented to the Advisory Committee on Immunization Practices (ACIP) on June 20, 2018.
Although vaccine performance can vary from season to season, flu vaccination remains the most reliable tool for preventing influenza and its complications. Each year, vaccination prevents millions of illnesses, millions of medical visits, and tens of thousands of hospitalizations. For example, during the 2016–2017 season, CDC estimates that vaccination prevented 5.3 million illnesses, 2.6 million doctor visits, and 85,000 hospitalizations. Similar estimates for 2017–2018 were scheduled for release in fall 2018.
Research has also shown that flu vaccination reduces the risk of flu-related death in children, reinforcing its value as a critical public health measure.
Was the 2017–2018 flu vaccine a good match for circulating viruses?
Yes. Most of the influenza viruses collected in the United States during the 2017–2018 season were found—both antigenically and genetically—to be similar to the cell-grown reference viruses used to produce the Northern Hemisphere vaccine for that year.
How many antiviral-resistant viruses were detected in 2017–2018?
Antiviral resistance occurs when flu viruses change in ways that make antiviral drugs less effective. From October 1, 2017, CDC tested 1,147 influenza A(H1N1)pdm09, 2,354 influenza A(H3N2), and 1,118 influenza B viruses for resistance to drugs such as oseltamivir, zanamivir, and peramivir. The majority of viruses remained susceptible; however, 11 (about 1%) of the H1N1pdm09 viruses showed resistance to oseltamivir and peramivir, though they remained sensitive to zanamivir. These results confirm that currently available antiviral drugs remain effective treatment options for circulating flu strains.