Every Monday, we’ll answer your questions on Covid-19 and health in a feature published online. You can submit a question here.

This week, HuffPost UK reader Robert asked: “Could I have had Covid in December?”

Covid-19 was officially confirmed in the UK on January 31, when two people believed to be Chinese nationals tested positive for the virus and were taken to hospital.

Since then, many have wondered whether the virus could’ve been circulating in the UK prior to that date – some experienced illness very similar to Covid-19 towards the end of 2019 but didn’t know at the time that the virus even existed.

So, what are the chances that the virus was circulating back then?

Dr Julian Tang, honorary associate professor in respiratory sciences and a clinical virologist at the University of Leicester, says it’s possible the virus was circulating in the UK in December 2019.

There were no restrictions to travel when the virus first started circulating and countries were unaware it even existed, so there’s no reason why it would’ve been contained in one place.

A study of the virus SARS-CoV-2, which causes Covid-19, analysed genetic samples from more than 7,500 people infected with it and suggested the pandemic actually started sometime between October 6 and December 11, 2019.

“Given that earlier Covid-19 cases in other European countries (Italy and France) have been found, it is very likely there were circulating cases in the UK amongst people who may have travelled to and from China in that period [between October and December],” Dr Tang tells HuffPost UK.

The South China Morning Post reported that Chinese authorities had traced its first Covid-19 case back to November 17. However, it wasn’t until January that the new virus was identified.

When asked how widespread the virus could have been in December in the UK, Dr Tang says it’s hard to say as the virus would have gone under the radar during a period when flu was also common.

In addition to anecdotal cases of people who suggest they had symptoms of the virus last year, there is also the case of Peter Attwood, 84, from Kent, who first experienced symptoms of the virus on December 28 and died a month later, on January 30.

The coroner ruled as recently as September 2020 that his cause of death was ‘Covid-19 infection and bronchopneumonia’, the Daily Mail reported. His daughter had become sick with a similar illness two weeks before her father developed symptoms – which would take us back to mid-December.

Sewage also tells us some more about the virus’ spread in Europe. Such systems are being tested around the world for traces of the virus – specifically, Covid-19 ribonucleic acid (RNA) – to help monitor its spread in populations. 

One study revealed the virus was present in Milan and Turin’s sewer systems as early as December 2019 – two months before the first positive cases in Italy. This was also before Chinese officials confirmed the virus existed.

In 2020, after the virus came to light, a French hospital retested samples from pneumonia patients and discovered that one of its patients had Covid-19 back in December. Professor Yves Cohen, head of the intensive care unit of the Avicenne hospitals in Bobigny, told BFM TV: “We had a positive case of Covid-19 on December 27.”

In the UK, less is known about the earliest cases without retesting archived blood and respiratory samples from that time for antibodies.