(NEXSTAR) — The omicron variant is forcing us to rethink what COVID-19 looks and feels like in its early days. While fever, coughing and loss of taste were the tell-tale early signs of the first strain of the coronavirus, early research suggests omicron manifests differently, especially early on in the illness.

One early symptom many are reporting is so mild and commonplace, it can be easy to miss: headaches.

A headache isn’t just one symptom of the omicron variant — it’s the second most common symptom, according to the ZOE Covid Study. The study is a joint effort created by researchers at Massachusetts General Hospital, the Harvard T.H. Chan School of Public Health, King’s College London, Stanford University School of Medicine and the health app ZOE.

The study found the only symptom more commonly reported by people infected with the omicron variant was a runny nose.

Similar findings came out of an early omicron case study in Norway, which tracked 81 people infected with the virus from a Christmas party outbreak. About 68% of the partygoers who contracted the virus reported headaches as a symptom.

Another study by the U.K.’s Health Security Agency of COVID cases in December found headaches to be a common symptom, as well. The study also found people who caught the delta variant also reported headaches.

This collection of common symptoms have made omicron more easily confused with a cold than past COVID strains, especially for breakthrough cases among the vaccinated.

Dr. Maya N. Clark-Cutaia, who teaches at the New York University Meyers College of Nursing, told the New York Times that vaccinated people’s omicron symptoms often present like a “really bad cold” — including headaches, body aches and a fever — while unvaccinated people are more likely to see flulike symptoms and shortness of breath.