However, 89 of the more than 11,000 hospital staff members who did not have antibodies - proteins produced by the human immune system to fight off infection - contracted the virus during the study period.
Medical staff members work in the Intensive Care Unit (ICU) where patients suffering from the coronavirus disease (COVID-19) are treated at the Melun-Senart hospital, near Paris, France, November 20, 2020.
However, the genetic material of the coronavirus can be detected for several weeks in both respiratory and stool samples, but it is not believed to be infectious.
The results were "consistent with Sars-CoV-2 re-exposure that did not lead to repeat symptoms", the study said.
It found that the level of virus causing the current pandemic is highest at the time symptoms begin, or before day five of symptoms.
Scientists have been scurrying to understand immunity against reinfection with SARS-CoV-2 (the virus that can lead to Covid-19).
In this study, the team of researchers at the La Jolla Institute of Immunology in California, and the Icahn School of Medicine at Mount Sinai in NY, investigated multiple compartments of circulating immune memory to SARS-CoV-2 in 185 Covid-19 cases, including 41 cases more than six months post-infection.
Lead author author Dr. Muge Cevik of the University of St. Andrews, U.K. said in a statement that the new analysis "provides a clear explanation for why SARS-CoV-2 spreads more efficiently than SARS-CoV and MERS-CoV and is so much more hard to contain".
Delirium is a common symptom of coronavirus in elderly patients, found in around one in three infected over-65s. The researchers said there appeared to be an increase in memory B cells over time.
Scientists had feared that those who developed only mild infections would be unlikely to have a strong immune response, but nearly all developed cells capable of creating new antibodies if they encountered the virus again.
The cohort had an average age of 78, and 84 (37 per cent) of the delirious patients did not have any typical Covid symptoms - such as a fever or shortness of breath.
The researchers concluded that it is unlikely that people remain infectious beyond this point.
However, overall prevalence of coronavirus infections rose to one in 80 people from one in 85 the previous week.
Katherine Kent, co-head of analysis for the Covid-19 Infection Survey, said: "There are early signs that the national level of infections in England might be levelling off but this hides a lot of variation at a regional level".
"In patients with non-severe symptoms, their period of infectiousness could instead be counted as 10 days from symptom onset", Cevik noted.
Study co-author, Dr Christoph Neumann-Haefelin, Head of the Gerok Liver Center at University Hospital Freiburg, also expressed optimism about their results, suggesting that immunity against the new coronavirus can be achieved after an infection, and that, "similarly, vaccines now being tested in trials could provide significant protection against SARS-CoV-2".
From these studies, the authors calculated the average length of viral RNA shedding and examined the changes in viral load and the success of isolating the live virus from different samples collected throughout an infection.