A paper published by the New and Emerging Respiratory Virus Threats Advisory Group said the duration of "natural or vaccine-induced immunity is not yet fully understood".
However, they added that immunity after getting a Covid-19 jab may last 90 days.
Scientists wrote: "Based on the variability in the data and differential responses in the population we conservatively estimate that a protective immune response after SARS-CoV-2 infection or vaccination may last for 90 days."
Covid 19 Time Series
Their estimate is based on the fact that other coronaviruses tend to reinfect after one to two years.
Reinfections with seasonal coronaviruses occur frequently at 12 months and sometimes as early as six months - but not within three months - scientists said.
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They explained: "Based on this information, we conclude that: Within one month of natural infection, a high proportion of people will develop immunity which is protective against disease caused by reinfection (high confidence).
"This protection is likely to persist for at least three months (moderate confidence).
"The level of protection against subclinical re-infection (as opposed to disease) is uncertain."
They said that 28 days after an effective vaccine, "a high proportion of people develop immunity which is protective against disease caused by reinfection (high confidence)."
Experts added that a small proportion of people may not develop immunity following either natural infection or vaccination - but the number is said to be small compared to the majority who get immunity.
According to NERVTAG, "natural immunity is sustained in most cases".
The document added that coronavirus antibodies can be detected in at least 90% of people who have been infected with the virus - and this figure approaches 100% with new tests.
The antibodies can be measured within about a week of symptoms developing, the paper added.
The document, which was published on Friday, was considered by SAGE on November 19.
Scientists said the trials of coronavirus vaccines "suggest that a high degree of immunity to COVID-19 disease can be obtained, at least in the short-term."
Despite some uncertainties around the level and duration of immunity, experts concluded: "Some form of Covid-19 immunity certification is likely to be possible but further data and considerations are needed before a recommendation can be made."