Viral Respiratory Infections

The influenza virus is a highly contagious respiratory pathogen that causes yearly seasonal epidemics and occasional pandemics, leading to significant health and economic consequences. It spreads through droplets, aerosols, and surfaces. Infected individuals are contagious before symptoms appear, with peak infectivity early in the illness. In the United States, the 2024-2025 influenza season was the most severe since 2017-2018, with an estimated 47 to 82 million cases, up to 1.3 million hospitalizations, and up to 130,000 deaths. The risk of adverse outcomes is higher for vulnerable groups, including older adults, infants, and individuals who are immunocompromised, those with chronic conditions, and individuals who are pregnant, long-term care facility residents, and individuals experiencing homelessness. Vaccination is the main preventive measure and is recommended for all patients 6 months and older unless contraindicated. Advances in vaccines have improved efficacy. For the 2024-2025 vaccine, efficacy ranged from 32% to 78%, depending on age group and outcome. Misinformation, limited access, mistrust, and disparities are barriers to administration. Antiviral medications including oseltamivir and zanamivir can reduce illness and complications if given early in the infection course, and new therapies are being developed for drug-resistant strains. Strategies to more effectively address influenza include improving access to vaccines, countering misinformation, performing community outreach, and advancing research on antiviral medications and universal vaccines.

On March 11, 2020, the World Health Organization declared COVID-19 a pandemic. By late 2024, COVID-19 had become endemic, with regional surges caused by new variants and declining population immunity. Case fatality rates have decreased because of vaccination and the emergence of less virulent variants, but long COVID persists as a public health concern. Because of structural issues, including those related to employment, housing, health care access, and mistrust, minority and low-income individuals are at higher risk for COVID-19 infection and mortality. Preventive strategies include shared decision-making about vaccination for individuals 6 months and older, two vaccine doses for adults 65 and older, and vaccination for pregnant and breastfeeding individuals. COVID-19 can be distinguished from influenza and respiratory syncytial virus infection through testing. Rapid antigen tests are faster but less sensitive than polymerase chain reaction tests. Outpatient care of COVID-19 focuses on symptomatic relief and early antiviral use when indicated. Inpatient treatment involves remdesivir, corticosteroids, interleuken-6 inhibitors, anticoagulation, and respiratory support. Long-term complications of COVID-19 include cardiovascular, pulmonary, neurologic, mental health, and renal conditions. Thrombotic risk peaks during acute infection and remains elevated for up to 1 year. An effective community response to COVID-19 requires equitable vaccine access, integrated care, and rebuilding of patient trust to counter misinformation.

Respiratory syncytial virus (RSV) causes bronchiolitis and other significant lower respiratory infections. It spreads mainly via droplets, direct contact, and contaminated surfaces. In most of the United States, the prevalence of RSV infection peaks during the winter. Annually, RSV causes approximately 58,000 hospitalizations and more than 2 million outpatient visits among US children younger than 5 years. It also causes up to 160,000 hospitalizations and 10,000 deaths in adults 65 years and older. Prevention comprises hygiene measures, avoidance of infected individuals, and vaccine and antibody strategies. Vaccination is recommended for adults ages 50 to 74 years with chronic health conditions or weakened immune systems or who live in long-term care facilities. Vaccination is also recommended for adults 75 years and older and pregnant women (between 32 weeks’ and 36 weeks and 6 days’ gestation). The long-acting monoclonal antibody nirsevimab is recommended for infants younger than 8 months born during or entering their first RSV season and high-risk infants and young children ages 8 to 19 months. Diagnosis of RSV infection is primarily clinical, with testing used for high-risk or hospitalized patients. Treatment is supportive. Hospitalization is needed for patients with hypoxia, dehydration, respiratory distress, or underlying high-risk conditions. Coinfections, socioeconomic disparities, and environmental factors (eg, tobacco smoke exposure) increase the risk of severe infection.