With the coronavirus vaccine race heading toward the finish line, many people are wondering what it will feel like to receive the longed-for drugs.

It’s the first time vaccinations in the U.S. have used mRNA technology, so experts still have many questions about the short- and long-term potential side effects.

Dr. Moncef Slaoui, the leader of the Trump administration’s COVID-19 vaccine program, Operation Warp Speed, said Pfizer and Moderna’s drugs are safe, but produced noticeable side effects in 10% to 15 % of people who received them in clinical trials.

According to CNBC, those who did suffer side effects reported redness and pain at the infection site, along with fever, chills, muscle aches, and headaches.

“The longer, more important kind of adverse events such as autoimmune disease or others have not been reported in a different way between the placebo group and the vaccine group in these two trials, which is very reassuring,” Slaoui said.

According to CNN, the vaccines use messenger RNA, or genetic codes, derived from the spike protein of SARS-CoV-2, the virus that causes COVID-19, to give the body instructions on how to fight the foreign invader. Both drugs require two doses. One, to get the immune system ready for action, and the second to boost the disease-fighting response.

One volunteer in the Moderna trials, 24-year-old Yasir Batalvi, said the initial shot was not bad and he suffered only localized pain at the injection site. The second dose, however, was different.

“I developed low-grade fever, and fatigue and chills,” he told CNN. He felt better the next day. Batalvi’s experience mirrors that of some of the Pfizer drug volunteers.

According to the New York Post, although the volunteers who took part in the phase 3 Pfizer trials weren’t told if they were injected with the vaccine or a placebo, the participants who experienced flu-like side effects such as muscle aches, fever, and headaches assumed they got the drug.

Glenn Deshields, 44, from Austin, Texas, suffered “severe hangover” symptoms that fortunately soon cleared. A post-vaccine antibody test that came back positive confirmed his suspicion that he did receive the vaccine and not the placebo, according to the Post.

Experts told CNN that feeling ill is not a sign that the vaccine gave you COVID-19, but that it is doing its job.

“That means your immune responds it working for you. You should feel good about that,” Dr. Paul Offit, a noted vaccine and infectious disease expert at the University of Pennsylvania told CNN. “And there shouldn’t really be any difficulty coming back for that second shot, knowing that  you’re now in a much better position to fight off this awful virus which has killed more than 250,000 people and can cause a lot of long term effects.”