Covid-19 was over five times more common among hospitalized people who were unvaccinated and had a previous infection, compared with those who were fully vaccinated and hadn’t had Covid-19 before, a study published by the Centers for Disease Control and Prevention found.

The report, released Friday and written by scientists from the federal agency as well as hospitals across the U.S., adds to the body of research suggesting that vaccines provide stronger protection against the coronavirus than prior-infection immunity.

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Covid-19 was over five times more common among hospitalized people who were unvaccinated and had a previous infection, compared with those who were fully vaccinated and hadn’t had Covid-19 before, a study published by the Centers for Disease Control and Prevention found.

The report, released Friday and written by scientists from the federal agency as well as hospitals across the U.S., adds to the body of research suggesting that vaccines provide stronger protection against the coronavirus than prior-infection immunity.

Research into immunity to Covid-19 has been limited in part by the short window of time in which the virus has been circulating and vaccines have been in use. Few clinical studies of patients who have recovered from Covid-19 have been conducted, compared with the more robust body of clinical research into the vaccines, making it hard to reach definitive conclusions about the nature of immunity provided by prior infection.

Some recent research suggests that while vaccines produce higher levels of antibody protection immediately after inoculation, the long-term protection provided by prior infection—via a type of long-lasting immune cells known as memory-B cells—may be more potent than the antibodies generated by vaccine-induced immune memory.

Researchers looked at 7,348 patients across the U.S. who had been hospitalized with Covid-19-like symptoms between the beginning of the year and early September.

Patients fell into two groups: 6,328 who had been fully vaccinated between three and six months before arriving at the hospital and had never had a previous Covid-19 infection, and 1,020 who had recovered from Covid-19 three to six months beforehand but remained unvaccinated.

The study showed that the unvaccinated, previously-infected group was 5.49 times more likely to have Covid-19 than the vaccinated patients who hadn’t been previously infected, after adjusting for various factors including age, geography and local virus circulation.

Federal health officials have said recently the durability and length of protection provided by previous infection are topics worthy of study. They recommend that people who have recovered from infection get vaccine shots as well to help break the chain of infection.

Among all 7,348 study subjects, “vaccine-induced immunity was more protective than infection-induced immunity against laboratory-confirmed Covid-19,” the study read. “All eligible persons should be vaccinated against Covid-19 as soon as possible, including unvaccinated persons previously infected with SARS-CoV-2.”

Write to Robbie Whelan at robbie.whelan@wsj.com