In some cases, levels of antibodies, or cells produced by the immune system to fight off infections, actually may increase over the course of the seven months, the data showed.
In addition, pre-existing antibodies against common cold coronaviruses could help protect against COVID-19, the researchers said.
"Cross-protection by pre-existing immunity to common cold coronaviruses remains to be confirmed," study co-author Carlota Dobaño said in a press release.
However, "this could help explain the big differences in susceptibility to the disease within the population," said Dobaño, a researcher with the Barcelona Institute for Global Health in Spain.
For this study, Dobaño and her colleagues tracked antibody levels in 578 Barcelona Institute for Global Health staff from the beginning of the pandemic in March 2020.
The researchers analyzed blood samples collected at four different timepoints between March and October 2020 and measured levels of three different antibodies against six known strains of COVID-19 as well as four coronaviruses that cause common colds in humans.
The majority of infections among healthcare workers in the study occurred during the first pandemic wave, with the percentage of participants with COVID-19 antibodies increasing to 16% in October from just under 14% in March, the data showed.
Those with antibodies had "stable" levels over the course of the study period, suggesting that they continued to produce antibodies long after their initial infections, the researchers said.
About 75% of the study participants with antibodies at seven months actually saw an increase in their levels of the immune cells "from month five onward, without any evidence of re-exposure to the virus," said Gemma Moncunill, a study co-author and Dobaño's colleague.
No cases of COVID-19 reinfections occurred among study participants, according to the researchers.
However, participants infected with COVID-19 had lower levels of human cold coronaviruses antibodies, particularly those who were asymptomatic, the researchers said.
Since the start of the pandemic, estimates as to how long those infected with the coronavirus remain immune to possible reinfection have varied from as little as three months to nine months.