Influenza & Respiratory Viral Infections in Adults

Influenza: presentation and testing

Clinical picture. After a 1–4-day incubation, influenza begins abruptly with high fever, chills, headache, prominent myalgias, dry cough, and sore throat. It spreads mainly by respiratory droplets; adults shed virus from about a day before symptoms until 5–7 days after onset, and young children and immunocompromised patients shed longer. Older adults may present with confusion, falls, lethargy, or an exacerbation of COPD or heart failure, sometimes without fever.

Testing. Test when the result will change management: hospitalized patients, high-risk outpatients, institutional residents, and infection-control decisions. Molecular assays (RT-PCR or rapid NAAT, often a multiplex panel with SARS-CoV-2 and RSV) are preferred; rapid antigen tests miss many cases, so a negative result does not exclude influenza. Intubated patients need a lower respiratory tract sample. Treatment of hospitalized and high-risk patients never waits for the result.

Antiviral treatment

Treat regardless of timing. Start an antiviral as soon as influenza is suspected in hospitalized patients, in anyone with severe or progressive illness, and in outpatients at high risk of complications, even beyond 48 hours; in these groups early treatment is associated with fewer complications and deaths. High-risk groups:

• age ≥65 years (and children younger than 5)
• pregnancy and the first 2 weeks postpartum
• chronic lung (asthma, COPD), heart, kidney, liver, hematologic, metabolic (diabetes), or neurologic disease
• immunosuppression from disease or drugs
• BMI ≥40; residence in a nursing home or long-term care facility; American Indian and Alaska Native people
• long-term aspirin therapy before age 19, because of the risk of Reye syndrome (acute liver failure with encephalopathy)

Healthy outpatients. Treatment started within 48 hours shortens illness by about a day and is reasonable, particularly for household contacts of high-risk people. After 48 hours, an improving low-risk patient needs supportive care: fluids, acetaminophen, and staying home until fever-free for 24 hours without antipyretics.

Prevention and infection control

Vaccination. Annual influenza vaccination is recommended for everyone 6 months and older; adults ≥65 preferentially receive a high-dose, adjuvanted, or recombinant vaccine. Inactivated vaccine is safe in any trimester of pregnancy, whereas the live attenuated intranasal vaccine is avoided in pregnancy and immunocompromise. Egg allergy is not a contraindication.

Chemoprophylaxis with once-daily oseltamivir supplements, but never replaces, vaccination:

• long-term care facility outbreak: all residents, vaccinated or not, for at least 2 weeks and until about a week after the last case
• exposed high-risk household or close contacts: start within 48 hours of exposure and continue for 7 days after the last exposure

Infection control. Hospitalized patients are placed on droplet precautions (private room, surgical mask) for about 7 days from onset or until 24 hours after fever and respiratory symptoms resolve, whichever is longer.

Complications of influenza

Secondary bacterial pneumonia. The classic course is biphasic: several days of improvement, then new fever, purulent sputum, and hypoxemia. S. pneumoniae is most common. S. aureus, including MRSA, causes necrotizing, cavitary pneumonia with hemoptysis, leukopenia, and shock; H. influenzae and group A streptococcus also occur. Obtain imaging and cultures, treat as community-acquired pneumonia, add vancomycin or linezolid when MRSA is suspected, and continue oseltamivir.

Primary influenza pneumonia. Relentless dyspnea and bilateral infiltrates from the start, without an interval of improvement, progress to hypoxemic respiratory failure and ARDS; pregnant women and patients with cardiopulmonary disease are most vulnerable.

Beyond the lungs. Myositis causes calf pain and tenderness, classically in children with influenza B. Rhabdomyolysis produces very high CK, dark urine, and pigment-induced AKI and is treated with aggressive isotonic fluids while avoiding NSAIDs. Myocarditis and pericarditis occur, and the risk of myocardial infarction rises sharply in the week after infection. COPD, asthma, and heart failure often decompensate, and encephalopathy and Guillain–Barré syndrome are rare neurologic complications.

RSV and other respiratory viruses

RSV in adults. Beyond infant bronchiolitis, RSV causes many winter hospitalizations of older adults and patients with heart or lung disease, with rhinorrhea, cough, wheeze, and COPD or heart failure exacerbations. Diagnosis is by PCR, and care is supportive: oxygen, bronchodilators, and treatment of the exacerbated condition. No antiviral is routinely used in immunocompetent adults.

COVID-19 in adults

High-risk outpatients. Mild-to-moderate COVID-19 in a patient at high risk of progression (older age, immunocompromise, chronic heart, lung, kidney, or metabolic disease, obesity, pregnancy) is treated with nirmatrelvir-ritonavir for 5 days, started within 5 days of symptom onset. Alternatives are a 3-day course of IV remdesivir started within 7 days, or molnupiravir when neither can be used (not in pregnancy). Brief symptom rebound after nirmatrelvir-ritonavir is usually mild and does not need retreatment.

Ritonavir is a strong CYP3A inhibitor. Hold simvastatin or lovastatin (and usually atorvastatin) during treatment; avoid coadministration with amiodarone, tacrolimus or cyclosporine, and rivaroxaban; adjust colchicine. CYP3A inducers such as rifampin, carbamazepine, phenytoin, and St John's wort make nirmatrelvir ineffective. When interactions cannot be managed, use remdesivir.

Hospitalized patients. Dexamethasone 6 mg daily for up to 10 days benefits patients who need supplemental oxygen but may harm those who do not. Remdesivir is used in hospitalized patients, particularly those on conventional oxygen, and baricitinib or tocilizumab is added for rapidly rising oxygen needs with systemic inflammation, alongside prophylactic anticoagulation.