COVID-19 and other respiratory viruses

• COVID-19 is the illness caused by SARS-CoV-2. It ranges from no symptoms to critical respiratory failure, and the risk of severe disease rises steeply with age and with underlying medical conditions.
• Treatment helps most when it is started early: antiviral tablets or infusions for people at higher risk should begin within 5 to 7 days of the first symptom. ^[1][2]
• RSV is the leading cause of hospitalisation in infants and a major cause of severe respiratory illness in older adults. COVID-19 and RSV now have effective vaccines or antibody products, while other common viruses are managed mainly with supportive care.
• The emergency warning signs of COVID-19 are trouble breathing, persistent chest pain or pressure, new confusion, inability to wake or stay awake, and lips, nail beds or skin that look pale, grey or blue.
• When a patient may have COVID-19 and is at higher risk, test early, start treatment early and give oxygen if hypoxaemic.

By the end of this page you should be able to:

1. Explain how SARS-CoV-2, RSV and human metapneumovirus cause respiratory disease and who becomes severely ill.
2. Recognise the presentations, warning signs and red flags of COVID-19, RSV and human metapneumovirus.
3. List the major risk factors for severe COVID-19 and severe RSV and the age cut-offs that matter most.
4. Choose the right antiviral and window for a higher-risk outpatient with COVID-19.
5. Apply the severity thresholds used for hospitalised COVID-19.
6. State the immunisation options for COVID-19, for RSV in adults, for RSV in pregnancy and for infants.
7. Explain what treatment exists, and what does not exist, for the other common respiratory viruses.
8. Adapt respiratory-virus care to a low-resource setting.

COVID-19 (coronavirus disease 2019) is produced by SARS-CoV-2, an RNA coronavirus that spreads mainly by the airborne and droplet route. This page treats COVID-19 as the central topic and then widens to the rest of the common respiratory viruses: RSV, human metapneumovirus, parainfluenza, adenovirus, rhinovirus, seasonal coronaviruses and others. The everyday clinical task is the same for all of them: distinguish a benign upper respiratory infection from severe lower respiratory tract disease and decide who needs treatment, oxygen or hospital care ^[1][4].

These illnesses form a spectrum shaped by the virus, the person's age and immunity, and the state of their lungs and heart. Guidance for COVID-19 has changed repeatedly since 2020; the newest open guidance used here includes the WHO clinical management living guideline (June 2025 edition) and CDC pages updated through October 2026 ^[5].

COVID-19 is infection with SARS-CoV-2, confirmed by detecting viral RNA or antigen in a respiratory specimen; reinfection can occur. Respiratory viruses are viruses whose main site of disease is the respiratory tract.

The common families are coronaviruses (including SARS-CoV-2 and the milder seasonal coronaviruses), orthomyxoviruses (influenza), paramyxoviruses and pneumoviruses (RSV, human metapneumovirus), parainfluenza viruses, adenoviruses, picornaviruses (rhinoviruses) and enteroviruses/bocaviruses. One practical split is upper versus lower tract disease (cold versus bronchiolitis, pneumonia, respiratory failure). Another is viral versus bacterial: viruses do not respond to antibiotics but predispose to secondary bacterial pneumonia.

COVID-19 and influenza cannot be distinguished on symptoms alone; testing separates them ^[3].

In temperate climates these viruses circulate in seasonal waves and often overlap. RSV usually peaks in December and January in the United States ^[7]. Human metapneumovirus spreads during late winter and spring, often alongside RSV and influenza ^[9]. COVID-19 has produced waves in all seasons.

In children under 1 year of age, RSV causes more bronchiolitis and pneumonia than any other virus, and it is the main reason infants are admitted to hospital in the United States ^[6][7]. It also causes substantial disease in older adults and in those with chronic heart or lung disease.

SARS-CoV-2 spreads by the airborne route; close indoor contact, crowding and poor ventilation raise transmission risk. Reinfection occurs as immunity wanes and variants change. Severity is driven by age and medical conditions rather than behaviour.

Conditions increasing the risk of severe COVID-19 are, per current CDC guidance: cancer; cerebrovascular disease; chronic kidney disease at any stage; chronic liver disease; chronic lung disease (moderate to severe asthma, bronchiectasis, bronchopulmonary dysplasia, COPD including emphysema and chronic bronchitis, interstitial lung disease including idiopathic pulmonary fibrosis, pulmonary embolism, pulmonary hypertension); cystic fibrosis; dementia or other neurological conditions; diabetes type 1 or 2; disability; heart conditions (heart failure, coronary artery disease, cardiomyopathies and possibly hypertension); haemoglobinopathies such as sickle cell disease or thalassaemia; HIV; immunocompromise; mental health conditions; overweight and obesity; physical inactivity; pregnancy (for at least 42 days after delivery); current or former smoking; solid organ or blood stem cell transplantation; substance use disorders; and tuberculosis ^[4].

Age dominates: more than 81% of COVID-19 deaths occur in people over 65, and deaths in those over 65 are 97 times higher than in those aged 18 to 29 ^[4]. Older age (65 years or more) is the strongest risk factor for severe COVID-19 (hospitalisation, intensive care, death), with risk rising as age rises; the risk also increases as the number of comorbidities increases ^[2][4].

For RSV, conditions that increase severe disease in adults aged 50 to 74 include chronic cardiovascular disease (heart failure, coronary artery disease, congenital heart disease, excluding isolated hypertension); chronic lung disease (COPD, emphysema, asthma, interstitial lung disease, cystic fibrosis); end-stage renal disease or renal replacement therapy; diabetes complicated by chronic kidney disease, neuropathy, retinopathy or other end-organ damage, or requiring insulin or an SGLT2 inhibitor; neurologic or neuromuscular disease impairing airway clearance or respiratory muscles; cirrhosis; chronic haematologic disease (sickle cell, thalassaemia); severe obesity (BMI 40 or above); moderate or severe immune compromise; nursing-home residence; and other risk factors including frailty ^[6].

In infants, severe RSV is most likely in premature babies, infants with weakened immunity, children with neuromuscular or congenital anomalies (including difficulty swallowing or clearing secretions) and those with severe cystic fibrosis ^[6].

SARS-CoV-2 infects ACE2-expressing respiratory epithelium. Symptomatic illness follows viral replication and the innate immune response. Severe COVID-19 arises from a mix of direct viral injury and dysregulated inflammation, with blunted early interferon responses in some patients and a later cytokine surge. In the lungs this gives diffuse alveolar damage, vascular leakage, microthrombosis and ARDS; systemic complications include cardiac, renal, hepatic, endocrine, gastrointestinal and neurologic injury and thromboembolism during prolonged hospitalisation ^[2].

RSV infects nasopharyngeal and bronchial epithelium, producing inflammation, mucus plugging and air trapping; in infants this manifests as bronchiolitis, while in older adults it destabilises COPD, asthma and heart failure. Human metapneumovirus is closely related and causes clinically similar disease, including pneumonia in older or immunocompromised adults ^[9].

Parainfluenza viruses commonly cause croup in young children and bronchiolitis or pneumonia in infants. Adenoviruses can cause febrile pharyngitis with conjunctivitis and, in some serotypes, severe pneumonia. Rhinoviruses dominate the common cold and trigger asthma exacerbations.

Severe COVID-19 shows diffuse alveolar damage, hyaline membranes, organising pneumonia, pulmonary capillary microthrombi and, in some cases, fibrosis, which explains the ground-glass imaging and delayed recovery. RSV produces bronchial epithelial necrosis, peribronchiolar and interstitial inflammation and, in severe cases, mucus plugging of small airways. HMPV and adenovirus can also injure lung parenchyma, so imaging may be atypical. Most rhinoviral and seasonal coronavirus colds stay superficial and spare the lungs, which is why they remain mild.

The spectrum runs from asymptomatic to respiratory failure. Common symptoms are fever or chills, cough, shortness of breath or difficulty breathing, sore throat, congestion or runny nose, new loss of taste or smell, fatigue, muscle or body aches, headache, nausea or vomiting and diarrhoea ^[3]. Symptoms may appear 2 to 14 days after exposure and may start mildly ^[3].

Emergency warning signs are: trouble breathing; persistent pain or pressure in the chest; new confusion; inability to wake or stay awake; and, depending on skin tone, lips, nail beds and skin that appear pale, grey or blue ^[3]. Anyone with these needs emergency care.

RSV most often causes a cold-like illness. In infants, rhinorrhoea and reduced appetite often come first, cough developing 1 to 3 days later, then sneezing, fever and wheezing; very young infants may have irritability, decreased activity or apnoea. Severe disease presents as bronchiolitis or pneumonia, sometimes in otherwise healthy young infants ^[6]. Two to 3% of infants under 6 months are hospitalised with RSV each year ^[6]. ^[7]

In adults RSV is usually mild (rhinorrhoea, sore throat, cough, headache, fatigue, fever, usually resolving in 1 to 2 weeks) but can be severe and hospitalising, and it can exacerbate asthma, COPD and heart failure ^[6]. It is a leading cause of infant hospitalisation and can be life-threatening in premature or high-risk infants ^[7].

HMPV causes upper and lower respiratory illness at all ages, usually mild in healthy children but severe in the very young, older adults and the immunocompromised. Symptoms are cough, fever, nasal congestion and shortness of breath, sometimes progressing to bronchitis or pneumonia; in adults it can complicate COPD ^[9]. It is most common from winter to spring and can circulate alongside RSV and influenza ^[9].

Parainfluenza classically causes croup in toddlers (barking cough, stridor) and bronchiolitis or pneumonia in infants. Adenovirus produces fever, pharyngitis and conjunctivitis and sometimes severe pneumonia. Rhinoviruses cause most colds and asthma flares. Seasonal coronaviruses cause mild colds in healthy adults but can be serious in infants, older adults and the immunocompromised.

The approach is to judge severity first, then cause. The table summarises the patterns.

Alternatives to a virus in a patient with fever, cough and sore throat include bacterial pneumonia, atypical pneumonia, tuberculosis, pulmonary embolism (including post-COVID-19 thrombosis) and exacerbations of heart failure or COPD. RSV and HMPV are clinically so similar that laboratory testing separates them; management is supportive either way.

COVID-19 is confirmed by NAAT (for example PCR) or antigen testing; multiplex panels can detect SARS-CoV-2 and influenza together, which is useful because the illnesses overlap clinically ^[3]. A positive test is not needed before treating a high-risk symptomatic patient ^[1].

Severity assessment in COVID-19 uses pulse oximetry, but oximeters may miss occult hypoxaemia, especially in people with darker skin, and smartphone oximeters may be inaccurate; closer monitoring is warranted in breathless patients ^[2].

In hospitalised infants or adults with RSV, PCR testing of a respiratory sample is preferred and antigen tests are used as rapid screens. HMPV is identified by molecular assays (preferred) or antigen tests ^[9]. General tests (full blood count, urea/electrolytes, C-reactive protein, imaging) are ordered by severity and local practice; imaging looks for complications such as pneumonia, not for the virus itself.

Severe illness is defined by any of: oxygen saturation (SpO2) under 94% on room air at sea level; PaO2/FiO2 ratio under 300 mm Hg; respiratory rate over 30 breaths per minute; lung infiltrates affecting over 50% of the fields; or the need for invasive mechanical ventilation ^[2].

In practice: mild illness means talking in full sentences and normal saturation on air; moderate illness may mean desaturation on exertion; meeting any threshold means severe disease needing hospital care, oxygen and protocol-driven treatment.

Critical illness comprises ARDS, sepsis or septic shock, and ventilation or vasopressor requirement. Organ support in intensive care is the core, and the WHO living guideline (June 2025 edition) is the regularly updated reference for hospitalised and critical care ^[5].

There is no universal score. RSV severity is judged by work of breathing (retractions, nasal flaring, grunting in infants), saturation, feeding or drinking ability, and apnoea or exhaustion. For HMPV, bronchiolitis and pneumonia in infants and frail older adults mark severe disease ^[9].

Most patients have asymptomatic, mild or moderate illness manageable at home ^[2]. Care comprises supportive and symptomatic treatment (antipyretics, analgesics, antitussives) for patients without severe-risk factors; clear advice on preventing transmission; explained red flags; and, for higher-risk patients, antivirals to significantly reduce hospitalisation and death, started within the first 5 to 7 days of symptoms, with close monitoring if declined ^[2].

Nirmatrelvir-ritonavir has been associated with lower hospitalisation and death in high-risk outpatients, and remdesivir is FDA-approved for COVID-19 treatment in adults and children ^[2]. Antiviral options are ^[1][2]:

IDSA recommends nirmatrelvir-ritonavir and remdesivir first-line for mild to moderate COVID-19 in at-risk people; molnupiravir is a second-line or alternative option ^[2].

Cautions: nirmatrelvir-ritonavir interacts with many drugs via CYP3A4, so check interactions before prescribing; remdesivir requires intravenous access over three days; and Paxlovid is no longer authorised for children aged 6 months to 4 years, for whom the only FDA-approved vaccine product is Moderna (Spikevax). Convalescent plasma with high-titre anti-SARS-CoV-2 antibodies can be used for patients with immunosuppressive disease or therapy ^[2].

Pemivibart (Pemgarda) is not a treatment but pre-exposure prophylaxis: an intravenous infusion over 60 minutes for people aged 12 years or older who are moderately or severely immunocompromised and weigh at least 88 pounds, with repeat doses every 3 months if continued protection is needed ^[1].

Hospital care includes oxygen, fluid and electrolyte balance and management of ARDS, sepsis, shock and thromboembolism. Patients may also suffer exacerbation of comorbidities or new organ disease ^[2].

Therapies for severe or critical disease include remdesivir and convalescent plasma as above, plus corticosteroids and immunomodulators according to severity and to WHO and national guidelines. Hospitalised regimens change frequently: follow the current local protocol. For ventilated patients, lung-protective ventilation, prone positioning and careful fluids are standard supportive principles. Secondary bacterial and fungal infections, including hospital-acquired pneumonia, should be treated if suspected.

COVID-19 rebound can occur 3 to 7 days after recovery or a negative test, with typically mild symptoms that resolve in a few days and no increase in severe illness in reported studies; it is not a reason to withhold antivirals in high-risk patients ^[1].

Management is supportive and most recover on their own. Infants need adequate hydration, suction and oxygen when needed; hospitalised infants may need oxygen, rehydration and, in severe cases, mechanical ventilation, usually improving within a few days ^[6]. No specific antiviral is licensed for routine RSV disease.

There is no specific antiviral and no vaccine for HMPV: care is supportive (fluids, rest, oxygen and airway support when needed) ^[9]. Treatment of parainfluenza, adenoviral and rhinoviral illness is likewise supportive. Products such as palivizumab are for prevention of RSV in selected high-risk infants, not for established infection.

Influenza has its own effective antivirals and vaccines and is treated in the influenza topic.

A clinical diagnosis suffices to start supportive care and COVID-19 treatment when testing is unavailable; do not delay treatment of a high-risk patient waiting for a test. Pulse oximetry is a high-value, low-cost tool for detecting silent hypoxaemia. Where antivirals are unavailable, prioritise oxygen, thromboprophylaxis where appropriate and clear referral criteria. In RSV season, teach caregivers the red flags (poor feeding, apnoea, retractions, grunting) and refer young or high-risk infants early. Oxygen delivery systems and referral pathways save more lives than sophisticated drugs in under-resourced facilities.

Severe and critical COVID-19 can produce: hypoxaemic respiratory failure and ARDS; sepsis and septic shock; inflammatory organ injury; pulmonary and systemic thromboembolism; hospital-acquired pneumonia; bacterial and fungal infection after prolonged ventilation or immunosuppression; and exacerbation of underlying chronic cardiac, pulmonary, renal, hepatic, endocrine, gastrointestinal or neurologic disease ^[2].

Long COVID (post-COVID conditions) can follow any acute infection, with prolonged fatigue, breathlessness, cognitive effects and reduced exercise tolerance; vaccinated people are at lower risk ^[2].

RSV can cause bronchiolitis and pneumonia and can worsen asthma, COPD and heart failure; RSV deaths occur mostly in the very young, the very old and the immunocompromised ^[6]. Severe HMPV can cause asthma exacerbation, wheezing, bronchiolitis, croup, COPD exacerbation and pneumonia in older or immunocompromised adults ^[9].

Multisystem inflammatory syndrome in children (MIS-C) is a rare serious complication of COVID-19 with fever, inflammation and multi-organ involvement, managed by paediatric specialists. Post-COVID patients benefit from early follow-up looking at breathlessness, oxygen, mental health and rehabilitation; persistent breathlessness has many causes (deconditioning, residual fibrosis, cardiac dysfunction, anaemia, pulmonary embolism) and should be assessed rather than assumed to be long COVID.

Most people with COVID-19 recover fully; death and critical illness are concentrated in older adults and those with underlying disease. Treatment of high-risk patients and vaccination both reduce hospitalisation and death; treatment reduces hospitalisation risk by more than 50% and lowers the risk of death ^[4]. ^[2]

RSV in healthy children usually resolves in a few days, while infants under 6 months carry substantial hospitalisation risk. RSV in adults is usually mild and self-limiting (1 to 2 weeks) ^[6].

Vaccination is the most effective preventive measure against severe COVID-19. For the 2026-2027 season, per CDC interim clinical considerations, a 2026-2027 COVID-19 vaccine is recommended for all adults aged 18 years and older; for children 6 months to 17 years with moderate to severe immunocompromise a vaccine is recommended, while for other children it is recommended through shared clinical decision-making. Products are mRNA and protein-subunit vaccines ^[8]. ^[1]

The same interim page notes that the Pfizer-BioNTech COVID-19 vaccine is no longer authorised for children aged 6 months to 4 years; the only FDA-approved product for that age group is Moderna (Spikevax) ^[8].

• Adults: CDC recommends RSV vaccination for all adults 75 and older, and for adults 50 to 74 at increased risk ^[6].
• Pregnancy: the maternal RSV vaccine (Pfizer's Abrysvo) is recommended at gestational weeks 32 to 36. ^[6]
• Infants: either maternal vaccination before birth or a long-acting infant antibody after birth (nirsevimab or clesrovimab). Nirsevimab is also recommended for a small group of children 8 to 19 months entering their second RSV season; clesrovimab is not recommended for that group ^[6].

Hand washing, covering coughs and sneezes, cleaning frequently touched surfaces, improving ventilation, staying home while symptomatic and, in healthcare settings, following isolation precautions reduce transmission of all respiratory viruses ^[9]. Because HMPV has no vaccine and no treatment, these measures are its main prevention ^[9].

Pregnancy. Pregnancy is among the conditions increasing severe COVID-19 risk; pregnant patients should be monitored closely and treated via the high-risk outpatient or inpatient pathway as appropriate. Drug safety in pregnancy must be checked against current labels. Pregnant women can receive the maternal RSV vaccine (Abrysvo) at weeks 32 to 36.

Children. Children usually have milder COVID-19, but severe disease, MIS-C and long COVID occur, especially with underlying conditions (medical complexity, genetic, neurological or metabolic disease, congenital heart disease, obesity, diabetes, asthma, sickle cell disease, immunocompromise). Infants under 6 months and those with prematurity or neuromuscular, immunologic or congenital conditions are at highest RSV risk ^[4][6]. Maternal vaccination or infant antibody prophylaxis is the main strategy against severe RSV in the first year.

Older adults. Adults 65 and over carry most COVID-19 mortality. Adults 75 and over should receive RSV vaccination, as should those 50 to 74 at increased risk. HMPV pneumonia is more likely in this group. COVID-19 vaccination applies to all adults, so older adults should be kept up to date annually. ^[1]

Immunocompromised patients. These patients are at high risk of severe COVID-19 and may respond less well to vaccines. Guidance includes additional vaccine doses, pemivibart prophylaxis (12 years or older, moderately or severely immunocompromised, at least 88 pounds) and convalescent plasma or antivirals when infected ^[1][4]. They are also high-risk for RSV, influenza and HMPV, so assess early.

Rural and remote communities. Where care or transport is difficult, health professionals may use clinical judgement, local RSV epidemiology and community hospitalisation rates to recommend RSV vaccination more broadly among adults 50 to 74 ^[6].

Refer urgently for any emergency warning signs of COVID-19; SpO2 under 94% on air or respiratory rate over 30; infants with RSV or HMPV who have apnoea, poor feeding, retractions or grunting, especially very young infants; pregnant patients with COVID-19 who are breathless or desaturating; immunocompromised patients with any respiratory infection; and suspected secondary bacterial pneumonia, sepsis, pulmonary embolism or MIS-C. Communication of diagnosis, severity, oxygen need and medications prevents duplication.

• Most respiratory infections are viral and self-limiting: avoid antibiotics without evidence of bacterial disease.
• Focus on recognising severity: hypoxaemia, work of breathing, inability to feed or drink.
• Oxygen therapy has the highest impact in severe COVID-19 and RSV; plan for power interruptions with spare cylinders and backup.
• Even simple supportive care, good feeding advice and home observation of red flags can reduce infant RSV mortality.
• Vaccine cold-chain storage and discussion of vaccination during routine care are the main operational tasks where programmes exist.

• COVID-19, caused by SARS-CoV-2, ranges from asymptomatic to ARDS, with severity rising sharply with age over 65 and with chronic disease.
• COVID-19 and influenza cannot be distinguished on symptoms alone; multiplex PCR separates them.
• Severe COVID-19 means SpO2 under 94% on air, PaO2/FiO2 under 300 mm Hg, respiratory rate over 30, infiltrates over 50%, or need for ventilation. ^[2]
• Offer antivirals to higher-risk outpatients within the first 5 to 7 days: nirmatrelvir-ritonavir within 5 days (first-line, with remdesivir), or molnupiravir within 5 days (second-line), or remdesivir within 7 days.
• Antiviral treatment reduces hospitalisation risk by more than 50% and lowers death risk.
• RSV is the leading cause of infant hospitalisation; 2 to 3% of infants under 6 months are hospitalised with it each year. It also worsens COPD, asthma and heart failure in adults. ^[6]
• RSV care is supportive: oxygen, rehydration and ventilation when needed.
• RSV immunisation covers adults 75+ (and 50-74 at increased risk), pregnant women at weeks 32 to 36, and infants via maternal vaccine or long-acting antibody.
• Human metapneumovirus, parainfluenza, adenovirus, rhinovirus and seasonal coronaviruses have no specific antiviral or vaccine (except influenza); their treatment is supportive and they co-circulate in winter and spring.
• COVID-19 vaccination is recommended for all adults 18+ and for children 6 months to 17 years with moderate to severe immunocompromise (shared decision-making for others).
• COVID-19 emergency warning signs: trouble breathing, persistent chest pain or pressure, new confusion, inability to wake or stay awake, pale/grey/blue lips, nail beds or skin.
• In low-resource care, recognising hypoxaemia and providing oxygen matter most.

• Influenza: clinically overlapping illness with its own antivirals and vaccines.
• Pneumonia and ARDS: complications of severe COVID-19 and RSV.
• Asthma, COPD and chronic respiratory disease: major risk factors.
• Bronchiolitis in infants: often caused by RSV.
• Sepsis and shock: complications of critical infection.
• Thrombosis and anticoagulation: a recognised complication of severe COVID-19.
• Vaccination principles and antiviral pharmacology.
• Paediatric respiratory infection: croup, bronchiolitis, wheeze.

Drug dosing and eligibility here follow current guidelines and product information as cited; local formularies, renal and hepatic function, pregnancy and drug interactions take precedence. This is an educational topic and not a substitute for clinical judgement or local guidelines.