Shannon Romano, a molecular biologist, met Covid in late March, about a week after she and her colleagues closed their laboratory at Mount Sinai Hospital. First there was a debilitating headache, followed by a fever that continued to rise, and then excruciating body aches. “I could not sleep. I couldn’t move, ”she said. “Each of my joints just hurts inside.”

It wasn’t an experience she wanted to repeat – never. When she was eligible for the Covid-19 vaccine earlier this month, she got the shot.

Two days after her injection, she developed symptoms that felt very familiar. “The way my head ached and my body ached was the same headache and body ache that I had when I had Covid,” she said. She recovered quickly, but her body’s intense reaction to the shock surprised her.

A new study could explain why Dr. Romano and many others who have had Covid report these unexpectedly intense reactions to the first shot of a vaccine. In a study published online on Monday, researchers found that people who were previously infected with the virus were more likely to report tiredness, headache, chills, fever, and muscle and joint pain after the first shot than those who had never been infected. Covid survivors had far higher levels of antibodies after both the first and second dose of the vaccine.

Based on these results, the researchers say, people who have had Covid-19 may only need one shot.

“I think one vaccination should be enough,” said Florian Krammer, virologist at the Icahn School of Medicine on Mount Sinai and author of the study. “It would also save people from unnecessary pain when they received the second dose, and it would release additional doses of the vaccine.”

While some scientists agree with his logic, others are more cautious. E. John Wherry, director of the Department of Immunology at the University of Pennsylvania, said before pushing for a policy change, he would like to see data showing that these antibodies were able to stop the virus from replicating. “Just because an antibody binds to part of the virus doesn’t mean it will protect you from infection,” he said.

It could also be difficult to determine which people were previously infected, he said. “Documentation is becoming a really potentially chaotic public health problem,” he said.

Side effects after vaccination are fully to be expected. They show that when the body comes into contact with the virus, the immune system is triggering a response and is better prepared to fight off an infection. Pfizer and Moderna vaccines are particularly good at producing a strong reaction. Most of the participants in the companies’ studies reported injection site pain, and more than half reported fatigue and headaches.

The clinical trials of Pfizer and Moderna’s approved vaccines, which each had more than 30,000 participants, suggest that most people experience the worst side effects after the second shock. And in the Moderna study, people who were previously infected actually had fewer side effects than those who hadn’t.

But anecdotally, researchers hear from a growing number of people like Dr. Romano feeling sick after being shot. “You describe these symptoms much more forcefully,” said Dr. Wherry.

This corresponds to what Dr. Krammer and his colleagues have found in their new study that has not yet been published in a scientific journal. The researchers assessed symptoms after vaccination in 231 people, of whom 83 were previously infected and 148 were not. Both groups widely reported pain at the injection site after the first dose. But those who had previously been infected were more likely to report fatigue, headaches, and chills.

The team also looked at how the immune systems responded to the vaccine in 109 people – 68 of whom were previously uninfected and 41 were infected – and found a more robust antibody response in the latter group. The numbers are small, however, and so the study’s conclusions need to be further explored with more research, experts said.

It is not necessarily surprising that previously infected people may experience more intense reactions. Both recordings contain genetic material that stimulates the body to produce spike proteins, the gnarled protrusions on the surface of the coronavirus. People who have already been infected with the virus have immune cells prepared to recognize these proteins. When the proteins show up after vaccination, some of these immune cells attack and people feel sick.

Dr. Susan Malinowski, an ophthalmologist in Michigan who had Covid-19 in March, undoubtedly felt that her body was attacked after receiving the Moderna vaccine. She got the first shot before lunch on New Years Eve. She was getting sick at dinner. She spent the next two days miserably in bed.

“I had a fever. I had the chills. I had night sweats. I was in pain all over my body, ”she said. “I was actually sicker after the vaccine than with Covid.”

Questions about more severe vaccine reactions in people who have already had Covid were asked at a meeting of an expert committee on January 27 advising the Centers for Disease Control and Prevention.

Covid19 vaccinations>

Answers to your vaccine questions

Am I eligible for the Covid vaccine in my state?

Currently more than 150 million people – almost half of the population – can be vaccinated. But each state makes the final decision on who goes first. The country’s 21 million healthcare workers and three million long-term care residents were the first to qualify. In mid-January, federal officials asked all states to open eligibility to anyone over 65 and adults of any age with medical conditions that are at high risk of becoming seriously ill or dying of Covid-19. Adults in the general population are at the end of the line. If federal and state health authorities can remove bottlenecks in the distribution of vaccines, everyone over the age of 16 is eligible as early as spring or early summer. The vaccine has not been approved in children, although studies are ongoing. It can take months before a vaccine is available to anyone under the age of 16. For the latest information on vaccination guidelines in your area, see your state health website

Is the Vaccine Free?

You shouldn’t have to pay anything out of pocket to get the vaccine, despite being asked for insurance information. If you don’t have insurance, you should still get the vaccine for free. Congress passed law this spring banning insurers from applying cost-sharing such as a co-payment or deductible. It consisted of additional safeguards prohibiting pharmacies, doctors, and hospitals from charging patients, including uninsured patients. Even so, health experts fear that patients will end up in loopholes that make them prone to surprise bills. This may be the case for people who are charged a doctor’s visit fee with their vaccine, or for Americans who have certain types of health insurance that are not covered by the new regulations. When you get your vaccine from a doctor’s office or emergency clinic, talk to them about possible hidden costs. To make sure you don’t get a surprise invoice, it is best to get your vaccine at a Department of Health vaccination center or local pharmacy as soon as the shots become more widely available.

Can I choose which vaccine to get?How long does the vaccine last? Do I need another next year?

That is to be determined. It is possible that Covid-19 vaccinations will become an annual event just like the flu vaccination. Or the vaccine may last longer than a year. We’ll have to wait and see how durable the protection from the vaccines is. To determine this, researchers will track down vaccinated people to look for “breakthrough cases” – those people who get Covid-19 despite being vaccinated. This is a sign of a weakening of protection and gives researchers an indication of how long the vaccine will last. They will also monitor the levels of antibodies and T cells in the blood of people who have been vaccinated to see if and when a booster shot might be needed. It is conceivable that people might need boosters every few months, once a year, or just every few years. It’s just a matter of waiting for the data.

Does my employer need vaccinations?Where can I find out more?

Dr. Pablo J. Sánchez, a committee member of the Research Institute at Nationwide Children’s Hospital in Columbus, Ohio, noted that he had heard from people who responded worse to the vaccine than their previous experience with Covid-19. He suggested adding a question about prior infection to the information the CDC requests from vaccine recipients. “It is not asked,” said Dr. Sánchez. “I think that’s really important.”

Dr. CDC’s Tom Shimabukuro, who provided safety data to the committee, said the agency is investigating the issue. “There is limited data on this at the moment, but we are looking at ways to get better information,” he said.

People who have had Covid seem to “respond to the first dose like it was a second dose,” said Akiko Iwasaki, an immunologist at Yale School of Medicine. So one dose is probably “more than enough,” she said.

A study published earlier this month reported that survival from natural infection offered 83 percent protection from re-infection within five months. “Giving two doses may seem excessive,” she added.

Shane Crotty, an immunologist at the La Jolla Institute for Immunology, pointed out that a more intense vaccine response typically means better protection. If someone had a big response to the first dose, “I would expect that skipping this second dose would be useful and that the second dose would likely be unnecessary,” he said.

However, other immunologists suggest that everyone stick to two doses. “I’m a big believer in getting the right dosage and timing because that’s how the studies were done,” said Maria Elena Bottazzi, an immunologist at Baylor College of Medicine in Houston.

And getting two shots doesn’t seem like a threat to those who have had Covid.

Nevertheless, the ophthalmologist Dr. Malinowski, there would be fewer questions and more answers. If the side effects of vaccines are really stronger in people who are already infected, health officials could give people a clue, she said.

“It would be nice to know that you might not be able to get up for two days,” said Malinowski. She decided not to come back for a second dose.

Dr. Mount Sinai Hospital’s Romano is expected to be shot for the second time in February and isn’t sure what she will do. “My friends who are immunologists, we all discussed this between us,” she said. “I will probably get it. But I want to think about it a little more before I do. “

Denise Grady and Apoorva Mandavilli contributed to the coverage.