Viral upper respiratory infection and influenza

01OverviewDefinition, clinical context and the essential points that orientate the chapter.

Upper-respiratory viral infection is a clinical syndrome of nasal obstruction or discharge, sore throat, cough and systemic symptoms caused by numerous viruses. Influenza sits on the same differential but has greater potential for systemic illness, pneumonia and deterioration in vulnerable people. COVID-19 cannot be separated reliably from symptoms alone.

Management begins with severity and risk rather than pathogen curiosity. Most low-risk patients need symptom care and safety-netting. Testing is valuable in hospitals, outbreaks, high-risk treatment windows and when infection-control action changes. Antivirals are targeted to influenza likelihood, timing, severity and risk group using current seasonal UKHSA guidance.

Clinical worsening needs a new diagnosis. Bacterial pneumonia, sinus or ear infection, myocarditis, encephalopathy, asthma or COPD exacerbation and dehydration can follow a viral illness. The correct response is examination and targeted investigation, not automatic extension of an antiviral or addition of antibiotics.

Key points

Most acute coryzal illnesses are viral and improve with supportive care; purulent nasal discharge alone does not prove bacterial infection or justify antibiotics.
Influenza more often causes abrupt fever, myalgia, headache, profound malaise and dry cough, but clinical features overlap with COVID-19 and other respiratory viruses.
Assess severity and host risk: pregnancy, older age, immune suppression and chronic cardiac, respiratory, renal, hepatic or neurological disease increase complication risk.
Use respiratory testing when it changes antiviral, isolation, outbreak or admission decisions; a negative result is interpreted against timing and sample quality.
Offer oseltamivir according to current NICE and UKHSA treatment criteria, ideally early, and do not delay in hospitalised or severely ill eligible patients solely for a result.
Antibiotics do not treat uncomplicated influenza or viral upper-respiratory infection; new focal consolidation, recurrent fever or clinical decline suggests bacterial complication.
Advise hydration, appropriate analgesia, infection-control measures and clear red flags; review vaccination after recovery rather than during the acute diagnostic decision.

02AetiologyUnderlying causes, associations and risk factors, with why each one matters.

Respiratory viruses

Rhinovirus, seasonal coronaviruses, influenza, respiratory syncytial virus and other viruses spread through respiratory particles, hands and contaminated surfaces.

Host susceptibility

Older age, pregnancy, immune suppression and chronic cardiac, pulmonary, renal, hepatic or neurological disease increase influenza complication risk.

Seasonal and exposure factors

Winter circulation, household crowding, healthcare outbreaks and incomplete current vaccination increase the probability of infection and transmission.

03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.

1
Mucosal epithelial infection
Viruses enter susceptible nasal, pharyngeal or respiratory epithelial cells, replicate and disrupt local barrier and ciliary function.
2
Innate inflammatory response
Interferons and cytokines produce congestion, sore throat, fever, myalgia and malaise while recruiting antiviral immune cells.
3
Lower-airway extension
Influenza and selected viruses can reach bronchi and alveoli, causing pneumonitis, impaired gas exchange and respiratory failure.
4
Secondary bacterial susceptibility
Epithelial injury and altered immune defence allow pneumococcus, staphylococci and other bacteria to cause pneumonia after initial viral illness.

04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.

Uncomplicated viral upper-respiratory infection

Rhinorrhoea, nasal congestion, sore throat and cough with preserved breathing, hydration and alertness usually support self-limited upper-airway disease.

Influenza syndrome

Abrupt fever, marked myalgia, headache, malaise and cough during circulating influenza increases probability, although atypical presentation occurs in older or immunosuppressed adults.

Lower-respiratory complicationRed flag

Dyspnoea, hypoxaemia, pleuritic pain, focal crackles or recurrent fever after initial improvement suggests viral pneumonitis or secondary bacterial pneumonia.

High-risk hostRed flag

Pregnancy, immune suppression, frailty and chronic organ disease lower the threshold for testing, antiviral treatment and same-day clinical review.

05InvestigationsWhat to request, why it matters and how to interpret it.

Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

01
Respiratory viral PCR or validated molecular testFirst step
Why
Identify influenza, SARS-CoV-2 or another virus when the result changes treatment or infection control.
Interpretation and limitations
Sensitivity depends on illness timing and collection. Detection can persist and coinfection occurs, so match the result to the syndrome.
02
Pulse oximetry and full observations
Why
Detect lower-respiratory involvement or physiological deterioration.
Interpretation and limitations
Normal resting saturation does not explain severe breathlessness automatically; exertional or serial change may reveal evolving disease in selected patients.
03
Chest radiograph
Why
Assess focal pneumonia, diffuse pneumonitis or another thoracic complication when lower-respiratory signs are present.
Interpretation and limitations
Imaging is unnecessary for routine coryza. An early normal film may not exclude evolving pneumonia when physiology deteriorates.
04
FBC, renal function and inflammatory markers
Why
Assess complication, dehydration and treatment modifiers in moderate or severe illness.
Interpretation and limitations
Biomarkers cannot reliably distinguish bacterial from viral infection alone. Renal function determines oseltamivir adjustment.
05
Bacterial cultures when complicated
Why
Identify secondary bacterial pneumonia or sepsis before antibiotics when feasible.
Interpretation and limitations
Sputum quality and prior treatment affect yield; collect blood cultures for severe systemic illness without delaying indicated antibiotics.

06Differential diagnosisRealistic alternatives and the features that help distinguish them.

COVID-19

Symptoms overlap substantially; current molecular testing and epidemiology rather than a single symptom distinguish SARS-CoV-2 when the result changes care.

Bacterial pneumonia

Focal consolidation, purulent lower-airway sputum, recurrent fever and bacterial microbiology support bacterial complication rather than uncomplicated upper-respiratory disease.

Streptococcal pharyngitis

Prominent tonsillar inflammation with validated clinical criteria and absence of viral features may support a bacterial sore-throat pathway.

Non-infectious airway disease

Allergic rhinitis, asthma, reflux and pulmonary embolism can cause cough or breathlessness without a primary acute viral infection.

07ManagementImmediate care, first-line treatment, alternatives and escalation.

01Low-risk upper-respiratory illnessUse supportive care and safety-nettingFirst stepSymptoms remain confined to the upper airway with stable observations, hydration and no high-risk host factor.

1Explain the likely viral course, review analgesic contraindications and advise fluids, rest, hand hygiene and respiratory etiquette.
2Avoid routine antibiotics and low-value testing, while considering COVID-19 or influenza testing when current public-health or treatment rules make it useful.
3Give a specific review trigger for breathlessness, dehydration, confusion, chest pain, recurrent fever or deterioration after initial improvement.

02Likely influenza in a high-risk patientStart the antiviral decision promptlyInfluenza is clinically or microbiologically plausible and severity, admission or risk factors meet current treatment guidance.

1Assess onset, pregnancy, immune status, renal function, swallowing and exposure and obtain respiratory testing if it will not delay indicated treatment.
2Start the current first-choice antiviral at the correct renal-adjusted dose and use infection-control precautions; seek specialist advice for severe immune suppression or treatment failure.
3Reassess for pneumonia, bacterial superinfection and organ complications if improvement does not occur as expected.

03Clinical deteriorationLook below the upper airwayNew hypoxaemia, focal signs, shock, confusion or recurrent fever develops during or after the viral syndrome.

1Perform ABCDE, repeat observations and examine for pneumonia, asthma or COPD exacerbation, myocarditis, pulmonary embolism and dehydration.
2Obtain chest imaging and targeted blood or microbiological tests, start sepsis or pneumonia treatment when criteria are met and continue indicated antiviral therapy.
3EscalationAdmit or escalate according to physiology and host risk, with a defined review of bacterial evidence and antimicrobial duration.

Key medicines and prescribing safety2 treatments · regimens, roles and cautions

Treats suspected or confirmed influenza in eligible at-risk, admitted or severely ill patients.

Oseltamivir

Usual adult treatment is 75 mg orally twice daily for five days, with renal adjustment and specialist extension for selected cases.

Adjust to current renal function and review vomiting and neuropsychiatric symptoms. Pregnancy is an influenza risk state and is not, by itself, a reason to withhold indicated treatment; use current UKHSA pregnancy guidance and seek specialist advice for severe immune suppression, complicated disease or treatment failure.

Relieves fever, headache, sore throat and myalgia during uncomplicated viral illness.

Paracetamol

Use 500 mg to 1 g orally up to four times daily, maximum 4 g in 24 hours for most adults.

Reduce maximum exposure in low body weight, liver disease, malnutrition or heavy alcohol use and check combination cold remedies to prevent inadvertent overdose.

08ComplicationsImportant consequences, why they occur and why they matter clinically.

Viral pneumonitis

Lower-airway inflammation may produce hypoxaemia, diffuse infiltrates and acute respiratory distress requiring admission and ventilatory support, especially in high-risk adults.

Secondary bacterial pneumonia

Damaged epithelium and altered immunity permit new consolidation, sepsis, pleural infection or necrotising disease after apparent initial improvement.

Cardiac or neurological injury

Influenza can precipitate myocarditis, encephalopathy, seizure, stroke or decompensation of chronic cardiac disease, causing disability beyond the respiratory illness.

Exacerbation of chronic disease

Asthma, COPD, heart failure and diabetes may destabilise during infection and require disease-specific treatment alongside antiviral care.

09Monitoring and follow-upTreatment response, safety checks and longer-term review.

Advise the patient to monitor breathing, hydration, alertness and trajectory rather than temperature alone, with a clear route for urgent reassessment.
For admitted or high-risk influenza, trend oxygen need, observations, renal function and ability to absorb oral treatment.
Review new focal chest findings, recurrent fever or inflammatory deterioration for bacterial pneumonia and culture before antibiotics when safe.
Check oseltamivir dose after any renal change and confirm intended course length in immune suppression or severe disease with specialists.
Report and manage institutional clusters through infection prevention and health-protection pathways, including staff and bed-movement implications.

10Special situationsVariants, exceptions and circumstances that change the usual approach.

Sputum colour is not proof

Neutrophils can make secretions yellow or green during viral illness, so colour alone does not establish bacterial infection.

Influenza can be afebrile

Older and immunosuppressed adults may present with confusion, weakness or respiratory decline without a dramatic temperature.

Timing matters but severity matters more

Antivirals work best early, yet hospitalised or severe eligible illness may still warrant treatment beyond a simple community time window.

Second deterioration is a clue

Initial improvement followed by new fever, dyspnoea or focal pain should trigger assessment for bacterial or inflammatory complication.

11Common pitfallsFrequent interpretation and management errors.

01
Prescribing antibiotics because nasal discharge or sputum is coloured.
02
Excluding influenza because fever is absent in a frail adult.
03
Waiting for PCR before treating severe eligible influenza.
04
Using oseltamivir without renal-dose review.
05
Attributing new hypoxaemia to a simple upper-respiratory infection.