Flu or COVID: When to Test and What Treatment Changes

On the first day a cold, the flu, and COVID can look identical: fever, aches, cough, sore throat. A test is worth doing when it can change what comes next, and what it changes is treatment: flu and COVID have antivirals that work best started early, an ordinary cold has none. If you are seriously unwell or in a higher-risk group, care does not wait for the result.

When to see a doctor

Right now — call emergency services. 911 in the United States and Canada, 999 in the United Kingdom, 112 across the EU; in Russia 112, or 103 for an ambulance. Non-urgent in the United Kingdom: 111.

An adult (CDC's flu list, which applies to respiratory infections generally): trouble breathing or shortness of breath; persistent pain or pressure in the chest or abdomen; persistent dizziness, confusion, or being impossible to wake; seizures; not urinating; severe muscle pain; severe weakness or unsteadiness; a fever or cough that improved and then came back or got worse; a chronic condition getting worse.
A child: fast or labored breathing, ribs pulling in with each breath; bluish lips or face; chest pain; severe muscle pain, to the point of refusing to walk; dehydration — no urine for eight hours, a dry mouth, no tears; not responding, or not waking normally; seizures; a fever above 40 °C that will not come down; any fever in an infant under 12 weeks; symptoms that improved and then got worse. The NHS (United Kingdom) adds a stiff neck; a rash that does not fade when pressed with a glass; dislike of bright light; a first febrile seizure; unusually cold hands and feet; pale, mottled, gray or blue skin; drowsiness and difficulty waking; extreme agitation. Neither list is exhaustive.

The same day — a doctor or a phone service. A fever lasting more than four days (CDC, United States) or more than three (NHS, United Kingdom); symptoms longer than ten days without improvement; severe facial pain or headache; signs of dehydration. For a child (NHS, United Kingdom): 38 °C under three months, 39 °C at three to six months, a fever of five days or longer, a rash with fever, refusing food and drink, dry diapers, sunken eyes.

Planned. A cough lasting more than three weeks (NHS, United Kingdom); repeated sinus infections; with a chronic condition or weakened immunity, a plan for the coming season agreed before it starts.

What is happening

The symptoms overlap, so the name of the virus comes from a test rather than from how bad you feel — and the reason to want the name is treatment. Antivirals have time windows counted from the first symptoms, and the windows differ from drug to drug, so there is no single "first two days" rule. That is also why "get tested" never means "wait for the result": CDC is explicit that in severe illness and in higher-risk groups, flu treatment starts without laboratory confirmation.

What to do

Check whether you are in a higher-risk group. CDC counts 65 and older; children under five, especially under two; pregnancy; chronic lung disease including asthma; heart disease; diabetes; chronic kidney disease; weakened immunity; disability with underlying conditions; a BMI of 40 or above. Then the plan is a doctor in the first days.
Flu. Four antivirals are in use: oseltamivir by mouth, zanamivir by inhalation, peramivir intravenously, baloxavir by mouth. Benefit is greatest when treatment starts within the first 48 hours; in severe illness, in hospital, and in higher-risk groups they are also given later.
Pregnancy. Oseltamivir is the preferred flu antiviral. Baloxavir is not recommended during pregnancy, during breastfeeding, or with severe immunosuppression.
Asthma or COPD. Inhaled zanamivir is contraindicated — it can cause bronchospasm. Respiratory infections are also a common asthma trigger: keep your action plan and inhaler within reach, and if you need the inhaler more and more, see a doctor the same day.
COVID, for people with risk factors. Nirmatrelvir/ritonavir within the first five days, from age 12 and 40 kg (CDC criteria, US); remdesivir within the first seven days, intravenously on three consecutive days; molnupiravir within the first five days, adults only, not in pregnancy. Ritonavir interacts with many medications, so the prescriber reviews what you take and checks kidney function.
Availability and prescription status differ by country — ask a local doctor or pharmacy.
A negative rapid test is not a green light if warning signs are present or you are in a risk group.

When this is not the case

For an ordinary cold there is nothing to start: no antiviral treats it and antibiotics do not act on viruses — antibiotics are not for viruses. The first days are eased the same way whichever virus it is: what helps with a cold.

Going back out is a separate question. CDC's guidance: at least 24 hours with no fever and no fever-reducing medicine, symptoms improving, then five more days of added precautions — mask, ventilation, distance. Local rules vary.

The tiers above outrank everything else here; the full checklist is on the hub, what to do at the first signs of a cold.

Sources

Checked: September 6, 2026