During the influenza season, when flu is circulating within the community, most people who get the flu experience self-limiting symptoms. However, severe disease can occur in older adults, in those with underlying medical conditions, and in the very young. The diagnosis of influenza is usually suspected based on characteristic clinical findings, particularly if influenza has been reported in the community.

Early diagnosis can reduce the inappropriate use of antibiotics and provide the option of using antiviral therapy. However, because certain bacterial infections can produce influenza-like symptoms, bacterial infections should be considered and appropriately treated, if suspected. In addition, bacterial infections can occur as a complication of influenza.

Although summer influenza activity in the U.S. is typically low, influenza cases and outbreaks can occur during summer months. Clinicians should consider influenza in the differential diagnosis of summer respiratory illnesses. Testing for seasonal influenza viruses and monitoring for novel influenza A virus infections should continue year-round (Xu et al., 2019).

Healthcare providers should also consider novel influenza virus infections in persons with influenza-like illness and swine or poultry exposure or with severe acute respiratory infection after travel to areas where avian influenza viruses have been detected. The local public health department should be alerted if a novel influenza virus infection is suspected.

Annual influenza vaccination is recommended for all persons aged ≥6 months and remains the most effective way to prevent influenza illness. Treatment as soon as possible with influenza antiviral medications is recommended for patients with confirmed or suspected influenza who have severe, complicated, or progressive illness; who require hospitalization; or who are at high risk for influenza-associated complications, including adults ≥65 years. Providers should not rely on less sensitive assays such as rapid antigen detection influenza diagnostic tests to inform treatment decisions. (Xu et al., 2019).

Laboratory Testing

The diagnosis of influenza is usually based on characteristic clinical findings, particularly if influenza has been reported in the community. Influenza virus testing is not required to make a clinical diagnosis but can inform clinical management when results may influence decisions to initiate antiviral treatment, perform other diagnostic testing, or implement infection and prevention control measures (Hall, 2021).

Diagnostic tests include:

• Molecular assays (i.e., rapid molecular assays, reverse transcription polymerase chain reaction (RT-PCR), and other nucleic acid amplification tests)
• Antigen detection tests (i.e., rapid influenza diagnostic tests and immunofluorescence assays)

In addition to diagnostic testing for only influenza virus, the Flu SC2 Multiplex Assay is a real-time RT-PCR test that detects and differentiates RNA from SARS-CoV2, influenza A virus, and influenza B virus in upper or lower respiratory specimens. Serology testing is no longer used for clinical diagnosis of influenza but is still used for research studies (Hall, 2021).

Flu Antiviral Agents

Antiviral medications are an important adjunct to influenza vaccine in the control of influenza. They work best when they are started within two days of getting sick. However, starting them later can still be beneficial, especially if the sick person is at higher risk of serious flu complications or is in the hospital with more severe illness (CDC, 2021a, August 31).

Antiviral treatment should start as soon as possible for adults and children with documented or suspected influenza—regardless of influenza vaccination history—who meet the following criteria (Uyeki et al., 2018):

• Persons of any age who are hospitalized with influenza, regardless of duration of illness.
• Outpatients of any age with severe or progressive illness, regardless of the duration of illness.
• Outpatients with chronic medical conditions and immunocompromised patients.
• Children younger than 2 years and adults ≥65 years.
• Pregnant women and those within 2 weeks postpartum.

Antiviral treatment should be considered for adults and children who are not at high risk of influenza complications, with documented or suspected influenza, irrespective of influenza vaccination history, who are

• Outpatients with up to 2 days illness onset before presentation.
• Symptomatic household contacts of persons at high risk of developing complications from influenza, especially those who are severely immunocompromised.
• Symptomatic healthcare providers of patients at high risk of developing complications from influenza, especially those who are severely immunocompromised (Uyeki et al., 2018)

During the SARS-CoV-2 pandemic, coinfection with influenza A or B viruses and SARS-CoV-2 should be considered. Influenza and COVID-19 have overlapping signs and symptoms. Studies have found that patients with COVID-19 who were coinfected with influenza shed SARS-CoV-2 longer than other patients with only COVID-19. Therefore, antiviral treatment is recommended as soon as possible, particularly for hospitalized patients with severe respiratory disease, outpatients with influenza-like illness, and patients with higher risk for influenza complications (Liu et al., 2021).

While flu vaccine can vary in how well it works, vaccination remains the best way to help prevent seasonal flu and its potentially serious complications. Antiviral drugs are a second line of defense that can be used to treat flu (including seasonal flu and variant flu viruses) (CDC, 2021a, August 31).

Four influenza antiviral medications approved by the FDA for use in the U.S. during the 2021–22 influenza season:

1. oseltamivir phosphate (available as a generic version or under the trade name Tamiflu®),
2. zanamivir (Relenza®)
3. peramivir (Rapivab®), and
4. baloxavir marboxil (Xofluza®). (CDC, 2021a, August 31)