COVID-19 Vaccines Halve Risk for Reinfection and Severe Disease COVID-19 Vaccines Halve Risk for Reinfection and Severe Disease

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For people who have recovered from a COVID-19 infection, vaccination reduces the risk for reinfection by half, compared with natural immunity alone, according to a new report.

Examining data from more than 18 million people across the world, Italian researchers also found that vaccination halves the risk of developing severe COVID-19 symptoms during a reinfection.

“Our results confirm that, among the recovered, those who have received two or three doses of vaccine have a 50% to 60% lower risk of reinfection than those who are not vaccinated,” study author Lamberto Manzoli, MD, a medical epidemiologist and director of the University of Bologna’s School of Public Health and Hygiene, Bologna, Italy, told Medscape Medical News.

“Considering that the number of people who recovered is now in the hundreds of millions worldwide, these results appear particularly encouraging and provide strategic information for future pandemic control policies,” he said.

The study was published November 9 in Frontiers in Medicine.

Calculating Risk

Manzoli and colleagues analyzed data from 18 studies conducted worldwide through July 2022. The studies included more than 18 million participants and evaluated the risk for COVID-19 reinfection among vaccinated and unvaccinated patients. The research team defined severe disease as requiring hospital admission without need of an intensive care unit and critical disease as requiring hospital admission in an intensive care unit or causing death. The studies were based in Asia, Europe, South Africa, and the United States and included the relevant COVID-19 vaccines authorized in those countries.

The research team evaluated several aspects of reinfection and severe disease, including the differences among people who received 1-3 vaccine doses, the lasting effects of protection 12 months after the last infection, and the severity and contagiousness of different variants. Most reinfections were reported during the Delta and Omicron waves.

Overall, the researchers found that vaccinated patients had a 53% lower likelihood of reinfection, compared with unvaccinated patients. Notably, they found similar levels of protection up to 12 months since the last infection. The results also didn’t change based on the different definitions of “reinfection” across the studies or based on the strain that predominated at the time of the study.

Fully vaccinated patients — those with both doses of a two-shot regimen or a single dose of a one-shot regimen — had slightly higher protection. They had a 55% lower risk for reinfection, compared with a 42% lower risk among partially vaccinated people who received one dose of a two-shot series.

Among patients who received three doses, or fully vaccinated people with a booster dose, the likelihood of reinfection was similar to that of the fully vaccinated people. People with three doses had a 54% lower risk of reinfection.

By variant, vaccinated people had a 60% lower risk of reinfection during the Delta wave and a 42% lower risk of reinfection during the Omicron wave.

Among seven studies with 2.3 million people that included information about disease severity, vaccinated people had a 55% lower likelihood of developing severe COVID-19 if reinfected.

Although the total number of reinfections during the COVID-19 pandemic may seem concerning, said Manzoli, cases with severe or fatal symptoms are relatively infrequent (fewer than 1 in 1000).

“It is worth noting that vaccines have reduced a thankfully already low risk,” said Manzoli. “These findings can thus be useful for planning specific immunization strategies for people who have already contracted the coronavirus.”

Additional studies are needed to understand the long-term persistence of protection, particularly with new COVID-19 variants, the study authors write. For now, the evidence appears to show stronger protection through hybrid immunity (that is, initial infection plus vaccination) vs natural immunity alone, they say, which may persist for a year and protect against the Omicron variant.

Making Future Plans

Commenting on the findings for Medscape, Joseph Hogan, ScD, a professor of public health and biostatistics at the Brown University School of Public Health in Providence, Rhode Island, said, “This information is crucial for formulating guidance about when to receive boosters. I continue to believe that high-quality data about vaccines will lead to high-quality decisions about their use as a preventive measure.”

Dr Joseph Hogan

Hogan, who wasn’t involved with this study, has researched COVID-19 vaccination and reinfection in the US, particularly among Rhode Island residents. He and his colleagues found that vaccination after recovering from COVID-19 reduced the risk of reinfection by 49% among long-term care residents, 47% among long-term care employees, and 62% in the general population when the Alpha and Delta strains were predominant.

“Vaccines continue to be one of the most important tools we have for preventing COVID infection. It’s easy for that message to get lost because many of us have COVID fatigue and frankly are tired of talking about pandemics,” he said. “But COVID infection still carries risk of serious illness and death, and preventing infection needs to remain high on the list of public health priorities.”

Hogan pointed to the high-quality data and methodology used in the analysis, as well as the consistent findings across the studies worldwide. For future studies, researchers are interested in looking at the differences by age group and the latest variants, including the use of genomic surveillance data to receive rapid estimates of vaccine effectiveness when new variants emerge, he said.

“One of my major concerns as a public health professional is that our federal government seems to be backing away from continuing funding and support for vaccines, testing, and treatment,” he said. “If access to vaccines becomes more difficult, we will be limiting the effectiveness of this and other important tools at our disposal.”

The study was conducted without commercial funding. Manzoli and Hogan report no relevant financial relationships.

Front Med. Published November 9, 2022. Full text

Carolyn Crist is a health and medical journalist who reports on the latest studies for Medscape, MDedge, and WebMD.

For more coverage of Italian medical news, visit Univadis Italy.