Dr. Anne Wyllie is a Microbial Disease Research Scientist and Epidemiologist from Yale School of Public Health. She developed SalivaDirect, which is an FDA-authorized salivary method for the detection of SARS-CoV-2.
During the pandemic, the public health measures advised for slowing the spread of SARS-CoV-2, the virus that causes COVID-19, also proved effective against the other common viruses that spread through our communities each year. While this resulted in markedly fewer reported cases of flu and of colds since early 2020, we’re seeing cases of colds and flu increasing once again as the same measures are relaxed and communities around the world reopen. With it almost certain that cases will increase even more during the next winter season, it’s important to understand the difference between the viral infections.
COVID-19, flu, colds, and even allergies can be hard to tell apart. Some of their symptoms appear similar in a number of ways, but there are some important differences that could help you identify the cause of your symptoms and the steps you should take to look after yourself and others.
While COVID-19 is caused by SARS-CoV-2, a novel coronavirus first detected in late 2019, flu is caused by either the influenza A or influenza B virus.  A “cold” is a generic term for an upper respiratory infection that can be caused by a number of different viruses, though most commonly rhinovirus.
Flu and colds generally have overlapping seasons from early fall through to early spring, and while human behavior is currently driving COVID-19 outbreaks around the world, it’s likely that future cases of COVID-19 will eventually also match the standard cold and flu season. This is because colder months drive individuals inside where there is less ventilation, making it easier for pathogens to spread between individuals. Additionally, the lower humidity can also make people more susceptible to infection.
Compared to flu, COVID-19 currently spreads more easily, but the infectiousness of both is generally greatest during the symptomatic period. COVID-19, flu, and colds all commonly spread through droplets in the air when someone who is sick talks, coughs, or sneezes. It is also possible to spread both a day or two prior to exhibiting symptoms. While cold and flu symptoms generally appear 1-4 days after exposure, COVID-19 symptoms generally appear in 2-5 days but in some cases could be as late as 9-12 days after exposure. As new variants of SARS-CoV-2 emerge, these rates of transmission and symptom onset can change. With Omicron, rates of transmission are proving higher and symptom onset can be as early as 1-3 days post exposure.
For more information about COVID-19 and flu, please visit the CDC website.
Many respiratory infections caused by either viruses or bacteria, can present with similar symptoms. Symptoms of flu and COVID-19 can be hard to tell apart, especially during the early stages and symptoms can range from mild to severe cases. Fever or chills, fatigue, and muscle aches may occur as your body starts to fight the infection. COVID-19 symptoms may also include headache, dry cough, shortness of breath, and stomach issues. However, headache and chest discomfort and cough are also common to flu. Loss of smell or taste are very characteristic of COVID-19 and are not seen in cases of flu. On occasion, COVID-19 may also cause rashes. Conversely, a runny and stuffy nose is less common with COVID-19 as compared to flu and colds. Sore throat and sneezing can also occur on occasion with flu.
Complicating the matter, SARS-CoV-2 variants can differ in their typical symptoms.  The most common symptom reported for Omicron is a sore throat early in infection, though it’s important to note that it is not the only one. More gastrointestinal symptoms have also been reported.
As for colds, fever, chills, and headache are rare. Aches and fatigue are also less common. Common symptoms are runny or stuffy nose, sneezing, sore throat, and chest discomfort or cough. Cold symptoms typically emerge more gradually, while flu symptoms appear more quickly.
It is important to distinguish these symptoms from allergies. Allergies are most common in spring and are typically caused by pollen that is released by plants. While pollen is almost invisible to the eye, as it spreads through the air, allergies can result as an immune response to them entering your body. As you generate antibodies trying to fight this invasion, they will release histamines into your system that can cause the symptoms of allergies, which appear almost immediately.
Itchy and watery eyes are generally unique to allergies, with itchiness possibly extending to the noses, sinuses, throat or even ears. Other symptoms include runny or stuffy nose, sneezing, cough, sore throat, and sometimes, headache, fatigue, and/or difficulty breathing . Allergies can be relieved with over the counter medications such as antihistamines and decongestants. This can help reduce symptoms such as itching, congestion, and sneezing. Eye drops may help to relieve itchy or watery eyes and a nasal spray can help decongest or ease inflamed sinuses. You may also want to test your immune response to indoor and outdoor allergens with Everlywell allergy tests.
Since symptoms can be hard to tell apart at times, getting tested is important for knowing the risk you pose to others and for helping to stop further outbreaks — even if you’re vaccinated. For either flu or COVID-19, it is important to isolate following a positive test so that you don’t pass the virus onto others who may be at greater risk of more severe symptoms than you. You may like to also notify any close contacts within the last week so they can also be on the lookout for symptoms and take additional measures to also protect others. Having tests available at home, or ordering ones that can let you sample yourself at home, can save yourself having to book doctors visits or come into contact with others.
Prevention is always the best strategy. Vaccinations for both flu and COVID-19 are available and protect against serious disease. Good handwashing for at least 20 seconds with soap and water has proven effective, especially after being out in busy areas or in places where you come into contact with numerous surfaces such as at shops, on public transport or even in the workplace. Hand sanitizer can also be used. It is important to always cover sneezes and coughs to limit spread of respiratory droplets. Using tissues or the bend of your elbow means you’re less likely to spread anything further than if you use only your hands. If you do use your hands, wash or sanitize them immediately after.
As reported cases increase during the annual cold and flu seasons, you may like to again wear your mask or even social distance a little to lessen your chance of infection from others. Importantly, do stay at home if you’re unwell. We have seen how effective these measures can be throughout the course of the COVID-19 pandemic, helping to not only curb the number of COVID-19 cases, but with other respiratory viruses mostly disappearing from the usual cold and flu seasons. Knowing how effective these could be throughout the winter seasons going forward — or whenever you may have symptoms — could do a lot to help protect those around you. While a cold, flu or even COVID-19 may turn out to be mild for you, you cannot be sure that it won’t affect somebody else more badly.
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