Influenza and COVID-19: Two Viral Respiratory Diseases

Influenza and COVID 19 are respiratory infections caused by different viruses. They can produce very similar symptoms, ranging from mild illness to pneumonia and organ complications. Testing can matter because antiviral treatment, infection control decisions and public health responses may differ.

Two illnesses caused by different viruses Influenza, usually called flu, is caused by influenza viruses. Seasonal illness in humans is mainly caused by influenza A and influenza B viruses. COVID 19 is caused by the coronavirus SARS CoV 2. It is not a type of influenza, despite sharing many symptoms with flu. Both viruses infect cells lining the respiratory tract. They can affect the nose, throat and larger airways or spread deeper into the lungs. Inflammation caused by either infection can affect the whole body. This helps explain fever, exhaustion, headache and muscle aches alongside respiratory symptoms. Knowing which virus is responsible may affect treatment, testing of contacts, outbreak management and access to antiviral medicines. Why symptoms overlap Both illnesses can cause fever, chills, cough, sore throat, headache, aching muscles, tiredness and reduced appetite. A blocked or runny nose can occur with either infection. Breathlessness, nausea, vomiting and diarrhoea are also possible. Gastrointestinal symptoms are particularly recognised in children with influenza. Symptoms alone cannot reliably distinguish flu from COVID 19. Other respiratory viruses, including respiratory syncytial virus and rhinovirus, can produce similar patterns. A person can occasionally have influenza and COVID 19 at the same time. Finding one virus does not always exclude another infection or a bacterial complication. The typical influenza pattern Influenza often begins suddenly. Someone may feel reasonably well, then develop fever, marked body aches, headache, dry cough and profound tiredness within hours. The illness often affects more than the nose and throat. Many people feel too unwell to continue normal activities. Most people with non severe influenza begin recovering within about one week. Cough, weakness and tiredness can persist after the fever settles. This pattern is characteristic but not diagnostic. Flu can be milder, especially after vaccination or previous exposure, and not everyone develops fever. The typical COVID 19 pattern COVID 19 can begin suddenly or gradually. Possible features include fever, continuous cough, sore throat, headache, fatigue, breathlessness and nasal symptoms. Loss or change of smell or taste can occur and may support suspicion of COVID 19. However, it is not present in every infection and is less prominent with some viral variants. Most people recover within a few weeks. Recovery time varies according to age, immune function, vaccination, previous infection and illness severity. Some people develop symptoms that continue or appear after the acute infection. This prolonged pattern is commonly called long COVID. Neither symptom pattern is absolute Sudden severe aching may make influenza more likely, while altered smell or taste may suggest COVID 19. Neither feature confirms the diagnosis. COVID 19 can resemble classic influenza. Influenza can resemble a cold, COVID 19 or another respiratory infection. Older adults and immunocompromised people may have less obvious fever or respiratory symptoms. New confusion, weakness or functional decline may be the first recognised change. Babies and young children may show reduced feeding, irritability, lethargy or breathing difficulty rather than describing headache or muscle pain. Clinicians therefore combine the illness pattern, examination, current virus circulation, risk factors and testing when it could change care. How the viruses spread Both infections spread mainly through respiratory particles released when an infected person breathes, speaks, coughs or sneezes. Transmission is more likely during close contact and in crowded or poorly ventilated indoor spaces. People may spread infection before recognising their symptoms. Hands and contaminated surfaces can contribute to transmission, particularly when respiratory secretions reach the eyes, nose or mouth. Opening windows, improving ventilation, covering coughs, disposing of tissues and washing hands reduce spread. A well fitting face covering can provide additional protection in crowded settings or around vulnerable people. People with a high temperature or who feel too unwell for normal activities should try to remain at home and avoid close contact with others. Who is at increased risk from influenza Most healthy people recover from influenza without hospital treatment. Certain groups have a greater risk of severe illness or complications. These include older adults, pregnant people, very young babies and people with chronic heart, lung, neurological, kidney or liver disease. Diabetes and significant obesity also increase risk. People with impaired immune function are more vulnerable. This includes some people receiving chemotherapy, transplant medicines, high dose steroids or other immune suppressing treatment. Pregnancy increases the risk of influenza complications, including pneumonia and hospital admission. Antiviral treatment should therefore be considered promptly when influenza is suspected. Who is at increased risk from COVID 19 The risk of severe COVID 19 rises with age. It is also increased by significant immune suppression and several long term health conditions. Higher risk groups include some people with cancer, organ transplants, advanced kidney disease, severe liver disease, neurological conditions or important immune disorders. Diabetes, heart failure, severe obesity and chronic respiratory disease can also increase the risk of deterioration. The effect of each condition varies between individuals. Vaccination and previous infection have reduced severe outcomes across the population. They do not remove risk completely, particularly in people with impaired immune responses. Eligibility for community antiviral treatment is based on current NICE and NHS criteria. It is not identical to vaccine eligibility or to every general risk factor for severe disease. Influenza complications Influenza can directly cause viral pneumonia. It can also damage respiratory defences and allow a secondary bacterial pneumonia to develop. Possible warning signs include worsening breathlessness, chest pain, returning fever or deterioration after an initial improvement. These features require reassessment rather than assuming the original flu is continuing normally. Severe influenza can cause sepsis, respiratory failure, encephalopathy or failure of several organs. It can also worsen asthma, chronic obstructive pulmonary disease and heart disease. Children can develop complications including ear infections, pneumonia, dehydration, febrile seizures or neurological illness. Most children do not develop these complications. COVID 19 complications COVID 19 can cause pneumonia and reduced oxygen transfer within the lungs. Severe inflammation may lead to respiratory failure requiring oxygen or ventilatory support. Hospitalised people can develop blood clots, kidney injury, cardiac injury or other organ complications. Risk depends on illness severity and the person's underlying health. A bacterial infection can occur alongside or after COVID 19, but it is not assumed from discoloured phlegm or fever alone. Children usually have milder acute COVID 19 than older adults. Severe respiratory illness and inflammatory complications can still occur, particularly in vulnerable children. Long COVID Long COVID describes symptoms that continue or develop after the initial COVID 19 infection. It can occur after mild or severe acute illness. Common features include fatigue, breathlessness, muscle or joint pain, altered smell, sleep difficulty and problems with memory or concentration. Symptoms may fluctuate and can affect several body systems. They should not automatically be attributed to COVID 19 without considering other diagnoses. Persistent or worsening symptoms require clinical assessment. Rehabilitation and management are based on the person's pattern and functional difficulties. Prolonged tiredness can also follow influenza. However, long COVID is a recognised multi system post infectious condition with a broader and sometimes prolonged course. Why testing can matter Because influenza and COVID 19 overlap clinically, laboratory or point of care testing may clarify which virus is present. A COVID 19 lateral flow test detects viral material associated with SARS CoV 2. A positive result supports current COVID 19 infection, while a negative result does not exclude every early or low level infection. People eligible for community COVID 19 treatment should test promptly when symptoms begin. Current NHS advice may include repeating lateral flow testing when initial results are negative but symptoms continue. Influenza can be detected using molecular tests or validated antigen tests. Multiplex tests can look for influenza A, influenza B, SARS CoV 2 and sometimes other respiratory viruses from one sample. Testing is particularly useful when the result will alter antivirals, hospital infection control arrangements or outbreak management. It may also help identify coinfection. Most otherwise healthy people with mild respiratory illness do not need extensive laboratory testing. Clinical services decide whether testing will meaningfully change care. Tests have limitations The accuracy of a respiratory test depends on the test type, timing, sampling quality and amount of virus present. Molecular tests such as PCR are generally more sensitive than rapid antigen tests. Greater sensitivity does not mean that a result answers every clinical question. A positive test identifies viral material but does not measure illness severity by itself. A person with a positive result can be mildly or critically unwell. A negative result may occur early in infection or after an inadequate sample. Clinicians may repeat testing or use another method when suspicion remains high. Severe influenza treatment should not be delayed while waiting for a result when the clinical and epidemiological pattern strongly supports influenza. Supportive treatment for both illnesses Most mild infections are managed with rest, fluids and treatment of discomfort. Paracetamol or ibuprofen may be suitable when taken according to instructions. Combination cold and flu products may already contain paracetamol. Taking these alongside separate paracetamol can cause an accidental overdose. Aspirin should not be given to anyone under 16 unless a specialist has prescribed it for a specific condition. Fluids help prevent dehydration. Reduced urine, dizziness, a dry mouth or increasing drowsiness suggests that intake may not be keeping pace with losses. People with significant long term conditions may need advice about regular medicines during acute illness. Medicines should not be stopped or doubled without appropriate guidance. Influenza antiviral treatment Antiviral medicines act against influenza viruses. They do not treat COVID 19 or bacterial pneumonia. Oseltamivir is the usual first line influenza antiviral for people at increased risk of severe illness and for people with severe influenza. It is taken by mouth or through a feeding tube when necessary. Zanamivir may be considered when oseltamivir cannot be used. Its inhaled form is not suitable for every patient, particularly when the person cannot use the inhalation device effectively. Influenza antivirals have the greatest benefit when started early, usually within 48 hours of symptom onset. Later treatment may still be valuable in severe, hospitalised or clinically deteriorating illness. A previously healthy person with mild, non severe influenza does not routinely need an antiviral. Clinical judgement may change this when there is a vulnerable household contact or another important concern. Pregnant people with suspected influenza are considered at increased risk. Current UKHSA guidance recommends oseltamivir as the usual first line antiviral when treatment is indicated. Influenza treatment should not await perfect certainty A clinician may start oseltamivir before a test result when influenza is strongly suspected in a high risk or severely unwell person. Treatment decisions consider symptom timing, local influenza circulation, illness severity and risk factors. A negative rapid antigen test may not safely exclude influenza. Kidney function affects oseltamivir dosing. Age, weight, pregnancy, swallowing ability and immune status can also alter the treatment plan. Influenza antiviral resistance is uncommon but monitored. Poor response in a severely immunocompromised person may prompt repeat testing and specialist virology advice. Antiviral medicines should not be borrowed, saved for another illness or used as a substitute for emergency assessment. COVID 19 antiviral treatment Most people with COVID 19 do not need a specific antiviral. The NHS offers early treatment to selected people at increased risk of progression to severe disease. Nirmatrelvir plus ritonavir is an oral antiviral option for eligible adults who do not need supplemental oxygen for COVID 19. Treatment must begin early in the illness, generally within five days of symptom onset. Nirmatrelvir blocks a viral enzyme needed for replication. Ritonavir slows the breakdown of nirmatrelvir so an effective concentration remains in the body. Ritonavir interacts with many commonly used medicines. A clinician or pharmacist must review prescriptions, over the counter products and supplements before treatment starts. Severe kidney or liver impairment and particular medicine combinations may make nirmatrelvir plus ritonavir unsuitable. Medicines must not be stopped independently to create apparent eligibility. Molnupiravir may be considered for eligible adults when nirmatrelvir plus ritonavir is unsuitable. Hospital treatments can include remdesivir and, for selected severe disease, corticosteroids or immune modifying treatment. Testing is linked to COVID 19 treatment access People who may qualify for community COVID 19 antivirals should keep current lateral flow tests available where advised by the NHS. They should test as soon as compatible symptoms begin, even if symptoms are mild. A positive result should be reported promptly to the GP surgery, NHS 111 or hospital specialist according to the local pathway. Treatment assessment considers age, health conditions, symptoms, oxygen needs, kidney and liver function and current medicines. A positive test does not mean that antiviral treatment is automatically appropriate. A negative test does not make worsening breathlessness or another red flag safe to observe at home. Local access arrangements can differ. The time sensitive nature of treatment means that people should not wait several days for routine review when they may be eligible. Antibiotics do not treat either virus Antibiotics do not kill influenza viruses or SARS CoV 2. They do not shorten uncomplicated flu or COVID 19. Antibiotics may be used when a clinician suspects or confirms a bacterial complication, such as secondary bacterial pneumonia. Discoloured phlegm does not prove bacterial infection. The overall clinical pattern, examination, tests and illness trajectory guide the decision. Unnecessary antibiotics can cause adverse effects and promote antimicrobial resistance. Leftover antibiotics should not be used for a new respiratory illness. Influenza vaccination Influenza viruses change over time, and vaccine protection reduces between seasons. The flu vaccine is therefore offered every year using vaccines selected for expected circulating strains. The NHS programme includes children, pregnant people, older adults and people in defined clinical risk groups. Care home residents, some carers, close contacts of immunocompromised people and frontline health or social care staff may also be included. Many children receive a live attenuated nasal vaccine. An injected inactivated vaccine is used when the nasal vaccine is unsuitable or for particular age and risk groups. Vaccination reduces the likelihood of severe influenza, hospitalisation and complications. It cannot prevent every infection because circulating strains and immune responses vary. The injected flu vaccines cannot cause influenza. The nasal vaccine contains weakened virus adapted to the cooler nasal passages and does not cause ordinary clinical flu. COVID 19 vaccination COVID 19 vaccination aims mainly to reduce severe disease, hospitalisation and death. Protection fades and SARS CoV 2 continues to change, so updated seasonal doses are offered to selected groups. For autumn 2026 and spring 2027, JCVI advised vaccination for adults aged 75 years or older, residents of older adult care homes and people aged 6 months or older who are immunosuppressed. Eligibility is narrower than the influenza programme because the programmes use different evidence, risk assessments and cost effectiveness decisions. A vaccinated person can still acquire and transmit COVID 19. Vaccination remains valuable because it reduces the risk of serious outcomes. Eligible people can usually receive influenza and COVID 19 vaccines during the same appointment. The exact products and schedules depend on age and clinical circumstances. Vaccination eligibility can change. Current NHS and UKHSA information should be checked each season rather than relying permanently on one age threshold. Vaccination does not diagnose or treat current illness A vaccine prepares immune defence before future exposure. It does not treat an influenza or COVID 19 infection that is already causing symptoms. Symptoms beginning shortly after vaccination may reflect expected vaccine effects, an unrelated infection or another condition. Timing alone does not identify the cause. People with a high temperature or who feel very unwell are usually advised to wait until they improve before vaccination. Mild illness without fever does not always require postponement. A previous infection does not provide guaranteed permanent protection. Reinfection with influenza or COVID 19 can occur as immunity reduces and viruses change. Reducing transmission Stay at home where possible when you have a high temperature or feel too unwell for normal activities. Avoid close contact with people at increased risk of severe infection. Improve ventilation by opening windows or using appropriate ventilation systems. Respiratory viruses accumulate more easily in crowded, poorly ventilated indoor spaces. Cover coughs and sneezes with a tissue or the bend of your elbow. Dispose of tissues promptly and wash your hands. Consider a well fitting face covering when contact with others cannot be avoided, particularly in healthcare settings or around someone who is vulnerable. Follow workplace, hospital, care home and public health instructions. These settings may use additional testing, exclusion or protective equipment measures. Recovery and reassessment Improvement from influenza usually includes settling fever, reduced aching and returning appetite. Cough and fatigue may take longer to resolve. COVID 19 often improves within several weeks, although some people recover sooner. Persistent symptoms do not automatically mean the person remains infectious. A new fever, worsening breathlessness or deterioration after initial improvement raises concern about pneumonia, another infection or a complication. A person with continuing breathlessness, chest discomfort, severe fatigue or cognitive symptoms should seek assessment rather than assuming prolonged recovery is harmless. Recovery expectations differ according to age, underlying health and illness severity. Comparison with another person's timeline is not a safe measure of progress. What this lesson should not be used for This lesson cannot determine whether a respiratory illness is influenza or COVID 19 from symptoms alone. It cannot interpret an oxygen reading or test result without clinical context. Do not use it to select oseltamivir, nirmatrelvir plus ritonavir or another antiviral. These medicines have eligibility criteria, timing requirements, dose considerations and important contraindications. Seek urgent help for breathing difficulty, chest pain, coughing blood, reduced responsiveness, severe childhood illness or rapid deterioration, regardless of the suspected virus.

Influenza and COVID 19 are caused by different viruses but have substantial symptom overlap. Influenza often begins suddenly with prominent fever and aching, while COVID 19 has a more variable course and can lead to long COVID. Testing matters when it changes antiviral treatment, infection control or outbreak management.