People who recovered from COVID-19 still had protection a year later, according to a new study.
Recovery from COVID-19 often grants a form of protection known as natural immunity.
Studies have consistently shown that severe COVID-19 induces natural immunity, but researchers in the Netherlands sought to see if people who did not require hospital care still experienced the protection.
They detected COVID-19 antibodies, or signs of a past infection, among 95 participants out of 497. About 70 percent had high levels of antibodies. Out of that subset, all but one remained “strongly seropositive” at all follow-up visits, which lasted up to one year.
Low rates of antibody decay were found, “suggesting long-term natural immunity,” Dymphie Mioch, a health researcher in the Netherlands and one of the researchers who carried out the study in the province of North Brabant, wrote with colleagues in the paper.
“The study verifies what we know about immunity after infection—it persists,” Dr. Jeffrey Klausner, an infectious diseases expert at the University of Southern California who was not involved in the research, told The Epoch Times in an email.
The study, funded by regional health authorities and the Dutch National Institute for Public Health and the Environment, was a prospective cohort study that involved analyzing blood samples from participants, who gave blood every three months for one year.
None of the participants were hospitalized.
Participants also filled out questionnaires, answering questions about their work, exposure, and history.
They were followed for up to one year or until they received a COVID-19 vaccine. The last followup was conducted in mid-2021.
Researchers analyzed the levels of total antibodies and Immunoglobulin G (IgG) antibodies. The presence of antibodies suggests the body is shielding against reinfection and severe disease.
Most studies examining natural immunity look at populations who experienced severe COVID-19, or groups of health care workers.
“However, such populations may not adequately represent the general population, while it is especially important for policymakers to have knowledge on the duration of protective immunity in the overall population,” the Dutch researchers said.
A smaller number of papers looked at the immune response to infection in generic populations, including U.S. researchers who found a response was detectable in more than 90 percent of participants for more than 5 months after infection, and Lithuanian researchers who found evidence of protection six months after infection in 95 percent of the individuals studied.
Those papers “report contrasting data regarding the duration of detectable antibody levels,” highlighting a need for longitudinal studies, or studies that examine people over time to measure changes, the Dutch researchers said.
The paper was published on Nov. 24 by the International Journal of Infectious Diseases.
More on Findings
Among the participants, 38 had decreasing levels of IgG antibodies and 44 did not. Most participants who did experience declines lost protection slowly. Eleven percent experienced a decrease of more than 50 percent in a three-month period; the majority had a decrease of 9 to 50 percent. Among the rest, an increase was recorded.
Across the entire year, IgG antibody declines were primarily recorded in the first six months.
For 37 people who completed all four visits, all had detectable antibodies on the fourth visit. Most had strong IgG antibody levels.
Researchers said they tried identifying a reason for the decay, but did not find one that was statistically significant. There were indications that people with a chronic disease saw the biggest declines, while women seemed to have drops less frequently.
Some studies have indicated that people who experience more severe cases of COVID-19 have higher levels of antibodies but researchers found the difference in their cohort was not significant when taking that factor, measured as who had a fever, into account.
“Although participants who reported fever generally had higher antibody levels in our cohort, these differences were not significant, potentially due to the small sample size or recall bias,” the researchers said.
“In conclusion, after an initial immune response due to a natural SARS-CoV-2 infection, we observed a reduction in antibody levels in approximately half of the studied seropositive hairdressers and hospitality staff in the Netherlands. However, most participants continued to have detectable antibody levels for up to one year,” they wrote. “Therefore, our real-world data results suggest long-term immune protection after natural infection. Future studies should investigate whether this detectable natural immune response also results in less (severe) reinfections after one year, especially considering that this virus is continuously evolving into new variants.”
Both natural immunity and vaccination can protect against severe illness, though studies indicate that the former is much more protective, particularly since Omicron started circulating in late 2021. Klausner, the California professor, noted that a number of papers have indicated that neither natural immunity nor vaccine-triggered protection shields for long against reinfection in the Omicron era.
“The challenge is that not all immunity is the same,” Klausner said. “Healthy people have strong immunity due to vaccination or infection that protects against serious or critical disease or death. That immunity however does not provide durable protection against infection. People can get reinfected 3–6 months after vaccination or recovery.”