Suspected cases of COVID-19 recorded by GPs at the height of the pandemic were three times higher than officially confirmed infections, according to new research.
The study suggests that coronavirus was more prevalent among the population than previously thought.
Many people who contracted COVID-19, including those with mild symptoms, will not have been tested, lead author Dr Sally Hull said.
Others may not have been able to access test centres.
Between 14 February and 30 April, GPs recorded 8,985 suspected cases, triple the number of people found positive at test centres over the same period.
The research, undertaken by Queen Mary University of London, discovered that people with dementia were seven times more at risk of developing a suspected case of the disease.
Black, Asian and minority ethnic (BAME) adults were twice as likely to present with suspected symptoms than white adults.
That increased risk was not necessarily because of factors such as other health conditions, obesity or social deprivation, the research found.
Unlike other studies, it discovered that women were at a slightly increased risk of a suspected infection compared to men.
Higher risks for men “emerge later in the disease trajectory”, the research suggested.
Suspected cases were studied because test results were not sent to GPs during the study period.
Anonymised data was taken from the primary care records of about 1.2m adults registered with 157 practices in four east London clinical commissioning groups during the peak of the outbreak.
Three of the four boroughs studied had death rates in the top five for London, while 55% of the population in those areas were from ethnic minorities.
Dr Hull said: “The high prevalence among BAME patients remains a big concern and we now know that ethnicity is still a risk factor even after you take account of social deprivation, long-term conditions and body mass index.
“So there is something else driving this, which urgently requires more research.”
Factors such as household size, employment, travel and the availability of personal protective equipment could not be taken into account.
Dr Hull added: “It’s going to be very important how GPs record and manage cases in their community, as this can provide an early warning system if cases are rising again in an area and if we’re about to see a second wave of infection.”
The study, which is peer-reviewed, has been published in the British Journal of General Practice.