Paxlovid, the antiviral drug used to treat COVID, isn’t the only reason you could be experiencing rebound symptoms, a new study suggests. In the study, published in JAMA Network in October, over a third of those infected with COVID who didn’t take Paxlovid still experienced a recurrence of their symptoms two days after their initial recovery.

The study, conducted between August and November of 2020, looked at over 150 people who did not take Paxlovid (they were given a placebo) after contracting the virus. After about a month, 68% recovered fully, and of that group, 44% reported they had recurring symptoms of at least one of 13 common COVID symptoms after two days of reporting zero symptoms (like chills, cough, fatigue, headache, muscle or body pain, nausea and sore throat).

The overwhelming majority (85%) of people with recurring symptoms said they were mild, and most commonly reported cough, fatigue, or a headache. Fifteen percent of those with recurring symptoms reported at least one moderate symptom.

“Reported symptoms are inherently subjective, and our observed variation may explain some of the rebound of symptoms after treatment for COVID-19, like in cases of what has been described as Paxlovid rebound,” the authors write.

People have feared taking Paxlovid, the antiviral treatment for COVID, because of the reported rebound effect, where people’s symptoms come back after they recover from the illness.

Researchers are still working to understand the rebound infection phenomenon, but many explain it as people’s immune systems not being able to fully fight off the virus yet.

“Your immune system hasn’t really had a chance to catch up and really create enough antibodies to have a sustained response,” Dr. Andrew Jameson, section chief of infectious diseases at Trinity Health Saint Mary’s Hospital, previously told Fortune.

Still, experts have touted Paxlovid’s ability to curb the risk of getting severely sick or dying from the virus—its main purpose. People who are eligible, those who are older or at risk for developing severe illness from COVID, are recommended to take the antiviral within five days of contracting the virus.

And for people who don’t take Paxlovid, the body’s immune response can still make rebound infection possible—everyone responds to the virus differently, and that includes recovering briefly and getting sicker a couple days later. Older people are more at risk for these rebound infections, Dr. Preeti Malani, an infectious diseases physician at The University of Michigan, tells Fortune.

“Recovery is not a straight line,” she says. “It just has to do with how your immune system takes care of the virus,” so expect symptoms to potentially increase and decrease after initial infection.

The study was conducted prior to the rollout of COVID vaccines, as well as the emergence of new variants like Omicron that have manifested differently across communities. Therefore, the results don’t take into account people’s current immunity from vaccination nor their response to different variants. While the thought of recovering only to get sick again is more than frustrating, experts underscore that there are things that can prevent severe illness. Those vaccinated are at much less risk for severe disease, hospitalization, and death.