Professor Luke O'Neill has warned that Long Covid-19 is a "real condition" that can affect people for several months.
However, he added it can now be more accurately diagnosed, and most people's symptoms clear within six months.
He told Newstalk: “How common (Long Covid-19) is is still uncertain, but it’s very clear it’s a real condition.
"There was a study in Oxford… brain fog is a key feature for maybe nine months post-infection. People have trouble concentrating."
He said health officials are now aware of "clear markers" of long Covid, making it easier to diagnose.
"They now have very clear markers of long Covid-19. There are two things in your blood: one is called interferon β, and another is called IL-6," he said.
"They go up in the blood of people with long Covid-19. That means you can diagnose it more accurately, and maybe treat it. Blocking those could relieve some of the symptoms."
And he shared the positive news about long Covid clearing up in most people after six months, saying people "haven't got symptoms forever".
He added: "The news is quite good. It does resolve in most people in about six months which is great. In other words, people haven't got symptoms forever which is a good sign.
"But it's still a concern because Omicron has been so widespread. So many got infected with Omicron there will still be a percentage of those that will have persistent symptoms for a few months anyway."
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