In

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In
to 650,000 deaths annually worldwide.
Aetiology\: caused by in
proteins (e.g. H1N1).
Transmission\: spreads via respiratory droplets, aerosolisation, person-to-person contact, or contaminated surfaces;
incubation 1-4 days.
Risk factors\: extremes of age, immunocompromise, comorbidities, pregnancy, healthcare workers, nursing home residents,
obesity.
Symptoms\: fever, severe headache, myalgia, cough, pharyngitis, rhinorrhoea, dyspnoea; GI upset common in children.
Investigations\: viral PCR is the diagnostic test of choice; CXR for respiratory illness requiring hospitalisation.
Management\: supportive care with rest, hydration, antipyretics, and analgesia; antivirals (oseltamivir or zanamivir) within 48
hours for high-risk or hospitalised patients.
Complications\: respiratory failure, ARDS, viral pneumonia, bacterial pneumonia, myocarditis, Guillain-Barre syndrome.
Vaccination\: annual
Article 🔍
A comprehensive topic overview

Introduction

In
an endemic infection which most commonly occurs during seasonal winter outbreaks.
1-2
In
2
In
3

Aetiology

In
in
2-3
In
classi
(e.g. H1N1).
1, 3
In
Yamagata and Victoria.
1, 3
In
epidemics each year and occasional pandemics.
3
TransmissionIn
sneezing or talking. It can also be transmitted via person-to-person contact and indirectly via contaminated surfaces.
2, 4-5
As such, it spreads easily within densely populated spaces and frequently causes outbreaks within schools, nursing homes
and hospitals.
4
The incubation period following exposure ranges from 1-4 days. Viral shedding peaks within 48-72 hours of exposure and
rapidly declines following day 6. Viral shedding is usually undetectable by day 10.
2, 5
Pathophysiology
In
surface molecules, enabling viral replication to take place.
3-5
Acute infection triggers a cascade of host responses, including the release of numerous pro-in
(notably the interferons) and defence mechanisms such as programmed cell apoptosis to minimise viral replication and
spread.
3-5
This results in mucosal in
columnar epithelium. Most in
lower respiratory tract infections can develop if the distal bronchial and alveolar epithelium are a
compromise progresses, acute respiratory distress syndrome (ARDS) and viral pneumonia can manifest.
1, 3-5

Risk factors

The risk factors for in
5, 7-8
Extremes of age (over 90% of deaths occur in over 65s)
Immunocompromise
Comorbidities (e.g. cardiovascular, respiratory, renal disease and diabetes)
Pregnancy
Healthcare workers
Residence in nursing homes or long-term care facilities
Obesity

Clinical features

The clinical presentation of in
tends to be more virulent than in
History
General malaise accompanied by severe headache, myalgia and fever is a typical presentation of in
1, 5
Other typical symptoms include\:
1, 5, 8
Cough
Pharyngitis
Rhinorrhoea
Dyspnoea
Lethargy
Gastrointestinal (GI) upset, including nausea, vomiting, and diarrhoea are common in children.
Clinical examination
In uncomplicated in
lymphadenopathy may be present.
5
If there is lower respiratory tract involvement, clinical
5, 8-9Vital sign derangement (e.g. hypoxia, tachycardia, fever, hypotension)
Respiratory distress (e.g. tachypnoea, unable to complete sentences, accessory muscle use)
Wheeze
Crepitations or bronchial breathing on auscultation

Di

Di
5, 8-9
SARS-CoV-2
Bacterial pneumonia
Respiratory syncytial virus
Rhinovirus
Parain
ARDS
Streptococcal pharyngitis
Pulmonary embolism

Investigations

Investigations should be guided by severity of the illness. Investigations beyond baseline observations are not usually
required for mild, uncomplicated illness.
Unless viral aetiology is con
infection.
Bedside investigations
Relevant bedside investigations include\:
Basics observations\: for hypoxia, tachypnoea, fever, tachycardia, hypotension and altered consciousness level
ECG\: for arrhythmia
Nasopharyngeal swabs should be obtained, with options including\:
5, 9
Viral rapid antigen test (RAT)\: high speci
Extended viral polymerase chain reaction (PCR)\: investigation of choice with high speci
may take up to 24 hours
Laboratory investigations
Relevant laboratory investigations include\:
Full blood count\: may show raised white cells or leukopaenia
Urea and electrolytes\: may demonstrate renal impairment
Liver function tests\: mild transaminitis is associated with acute viral infection
C-reactive protein\: a modest rise in CRP is consistent with in
Arterial blood gas\: if high risk of type 2 respiratory failure (T2RF) or acutely hypoxic
Imaging
All patients with an acute respiratory illness requiring hospitalisation should receive an urgent chest X-ray (CXR). CXR may
demonstrate peripheral pulmonary in
5, 9
If CXR is inconclusive, CT thorax can be considered.Figure 1. Chest X-ray in an in
demonstrating bilateral peripheral in
consolidation

Diagnosis

The diagnosis of in
cases do not require further investigation. The pre-test probability is higher during periods when in
circulating, such as seasonal winter epidemics or during acute outbreaks in healthcare facilities.
2, 5
When indicated, diagnosis is con
viral RATs are available in most supermarkets and chemists.

Management

Conservative management
Management is primarily supportive for uncomplicated illnesses. Patients should be encouraged to rest and maintain
good hydration, with antipyretics and simple analgesia for symptom control.
5
Acute management
All acutely unwell patients should be assessed and managed with a thorough ABCDE approach.
Acute management may include\:
Supplemental oxygen\: if hypoxic, aiming >94% or 88-92% if at risk of T2RF
Bronchodilators\: if wheezy or underlying obstructive airway disease
Analgesia\: for myalgia, headache etc
Antipyretics\: for fever
Intravenous (IV) \: for dehydration and acute hypotension
Venous thromboembolism (VTE) prophylaxis\: all hospitalised patients should have a VTE risk assessment
Antibiotics\: if superadded bacterial infection is suspected
Antivirals
Antivirals are recommended for hospitalised patients and those at increased risk of progression to severe disease.
5
The use of antivirals is controversial. Evidence has demonstrated that the early administration of antivirals produces a
modest reduction in symptom duration. Observational evidence suggests that there may be a slight reduction in mortality
risk.
5, 10-11
For suitable patients, NICE recommends neuraminidase inhibitors, which should be initiated within 48 hours of symptom
onset. Treatment initiation after 48 hours is o
5, 11
NICE recommends\:
5, 11-13
Oral oseltamivir (Tami®) 75 mg twice daily (BD) for 5 days, or
Inhaled zanamivir 10 mg BD for 5 days (if oral administration not tolerated)
For life-threatening illness\:IV zanamivir 600 mg BD for 5-10 days
Post-exposure prophylaxis
Post-exposure prophylaxis (PEP) should be o
unvaccinated or within 14 days post-vaccination.
5, 11
For PEP, NICE recommends\:
5, 11-13
Oral oseltamivir 75 mg once daily (OD) for 10 days, or
Inhaled zanamivir 10 mg OD for 10 days
Vaccination
Vaccination is considered the best method of preventing in
observed with in
For 2024-2025, the UK in
5, 14
Children aged 2-3 years old
School-aged children in reception-year 11
Aged six months-65 years old with comorbidities
All those over 65 years old
Pregnant women
Residents of long-term care facilities
Full-time carers
Healthcare professionals
Immunocompromised individuals
Contacts of immunocompromised individuals

Complications

Complications of in
5, 8
Respiratory failure
ARDS
Viral pneumonia
Superadded bacterial pneumonia
Otitis media
Cardiovascular complications (e.g. myocarditis, pericarditis)
Guillain-Barre syndrome
Pregnancy complications (e.g. perinatal mortality, pre-term labour)

References

StatPearls. InLINK].
World Health Organisation. InLINK].
Taubenberger JK, Morens DM. The Pathology of In
Disease. 2008 Aug 11;3(1)\:499–522.
Brankston G, Gitterman L, Hirji Z, et al. Transmission of in
Apr;7(4)\:257–65.
BMJ Best Practice. InLINK].Flerlage T, Boyd DF, Meliopoulos V, et al. In
respiratory tract. Nature Reviews Microbiology. 2021 Apr 6;19(7)\:1–17.
Gaitonde D, Moore F, MacKenzie M. In
NICE. InLINK].
Medscape. InLINK].
Gao Y, Guyatt G, Uyeki TM, et al. Antivirals for treatment of severe in
of randomised controlled trials. The Lancet. 2024 Aug 1;404(10454)\:753–63.
BNF. InLINK].
BNF. Oseltamivir. Published in 2024. Available from\: [LINK].
BNF. Zanamivir. Published in 2024. Available from\: [LINK].
GOV.UK. Flu vaccines for the 2024 to 2025 season. Published in 2024. Available from\: [LINK].
Image references
Figure 1. Weerakody Y et al. Radiopaedia. InLINK]. License\: [CC BY-NC-SA 3.0].

Reviewer

Dr Ali Khan
Consultant in Infectious Diseases and General Medicine

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Contents

Introduction
Aetiology
Risk factors
Clinical features
Source\: geekymedics.com