Flu Season 2020: 9.3M to 45M Americans fall sick and 12,000-61,000 die each year, says CDC
During the last season, the rates of hospitalization in adults aged 18-49 years were the highest seasonal rates seen since 2017-18
The burden of the flu disease in the US can vary widely and is determined by several factors, which include the characteristics of circulating viruses, the timing of the season, how well the vaccine is working to protect against illness, and how many people got vaccinated. But while the impact of flu varies, it places a substantial burden on the health of Americans each year, say health experts.
The US Centers for Disease Control and Prevention (CDC) does not know the exact number of people who have been sick and affected by flu because it is not a reportable disease in most areas of the US. Flu deaths in adults are also not nationally notifiable. However, the agency estimates that influenza has caused 9.3 million to 45 million illnesses, 4.3 million to 21 million medical visits, 140,000 to 810,000 hospitalizations, and between 12,000 to 61,000 deaths annually since 2010.
While seasonal influenza viruses are detected year-round in the US, flu viruses are most common during the fall and winter. The exact timing and duration of flu seasons can vary, but influenza activity often begins to increase in October. Most of the time flu activity peaks between December and February, but activity can last as late as May.
A 2018 CDC study looked at the percentage of Americans who were sickened by the flu using two different methods and compared the findings. Both methods had similar findings, which suggested that on average, about 8% of the US population gets sick from the flu each season, with a range of between 3% and 11%, depending on the season. “The commonly cited 5% to 20% estimate is based on a study that examined both symptomatic and asymptomatic influenza illness, which means it also looked at people who may have had the flu but never knew it because they didn’t have any symptoms. The 3% to 11% range is an estimate of the proportion of people who have symptomatic flu illness,” write researchers.
The analysis, published in Clinical Infectious Diseases, also found that children are most likely to get sick from the flu. Median incidence values (or attack rate) by age group were 9.3% for children 0-17 years, 8.8% for adults 18-64 years, and 3.9% for adults 65 years and older. This implies that children younger than 18 are more than twice as likely to develop a symptomatic flu infection than adults 65 and older.
2019-20’s ‘moderate’ season caused 38 million illnesses
Estimates of the 2019-20 season suggest that that the burden of illness was ‘moderate,’ and it was associated with 38 million illnesses, 18 million medical visits, at least 400,000 hospitalizations, and 22,000 deaths. “These numbers are estimated using a mathematical model, based on observed rates of laboratory-confirmed influenza-associated hospitalizations,” the study explains.
While the 2019-20 season is described as having moderate severity, the effect of influenza differed by age group, and the severity of the season in some age groups was higher. The burden was higher in young children (0-4 years) and adults (18-49 years) compared with the 2017-2018 season, a recent season with high severity, and provides evidence to support how severe seasonal influenza can be at any age, explains the CDC. More than 52,000 hospitalizations occurred in children less than 18 years and 86,000 hospitalizations among adults aged 18-49 years. “The estimates of deaths and hospitalizations show the effects that influenza virus infections can have on society. The rates of hospitalization in adults aged 18-49 years were the highest seasonal rates seen since the 2017-2018 season. These rates mean that an estimated 15 million cases of influenza in younger adults (18-49 years), which is the highest number of infections for this age group since CDC began reporting influenza burden estimates in the 2010-11 season,” says the analysis.
Flu activity in the US during the 2019-2020 season began to increase in November and was consistently high through January and February. The season was characterized by two consecutive waves of activity, beginning with influenza B viruses and followed by A(H1N1)pdm09 viruses. Overall, influenza A(H1N1)pdm09 viruses were the most commonly reported influenza viruses the last season. Activity began to decline in March, perhaps associated with community prevention measures for Covid-19, according to experts.
About 43% of hospitalizations occurred in older adults aged 65 years or older. Older adults also accounted for 62% of deaths, which is lower than recent previous seasons. “These findings continue to highlight that older adults are particularly vulnerable to severe disease with influenza virus infection. An estimated 7,800 deaths (36% of all deaths) occurred among working-age adults (aged 18-64 years), an age group for which influenza vaccine coverage is often low. This also underscores that influenza viruses can affect individuals of any age and prevention measures such as vaccination are important to reducing the impact of the seasonal epidemics on the population and healthcare system,” the team cautions.
Flu deaths reached ‘tragic milestone’ last season
Flu deaths in children have been nationally reportable since 2004. Since then, flu-related deaths in children reported to CDC during regular flu seasons have ranged from 37 to 188 deaths. For comparison, during the 2009 H1N1 pandemic, from April 2009 to September 2010, 358 flu deaths in children were reported to the agency.
In August this year, the CDC announced one new seasonal flu-related death in a child that occurred during the 2019-2020 season, bringing the total number of reported flu deaths in children for that last season to 188. This matches the highest recorded number for pediatric flu deaths reported during a regular flu season, which occurred during the 2017-2018 flu season.
Among the 188 pediatric flu deaths, 43% (81) occurred in children younger than 5 years old, 12 occurred in children younger than 6 months and thus too young to get a flu vaccine, and 57% (107) deaths occurred in children 5-17 years old. Of the 175 pediatric deaths among children with known information on medical conditions, 76 (43.4%) had a pre-existing medical condition. Nearly two-thirds of the deaths were attributed to influenza B infections. “This tragic milestone underscores how serious flu can be and serves as a reminder of the importance of an annual flu vaccine. In 2019-2020, only 21% of children eligible for vaccination were fully vaccinated against the flu. This percentage was similar to past seasons,” says the CDC.
The agency also emphasizes that the number of pediatric flu deaths reported to it each season is likely an undercount. For example, even though the reported number of deaths during the 2017-2018 flu season was 188, CDC estimates the actual number was closer to 600. It is likely the actual number of children who died from flu during the 2019-2020 season is higher as well, the agency suggests.