London: A new study in the UK has revealed that while many people have reported after effects of the Pfizer Covid vaccine, the most common symptoms are of a non-serious nature.

Most symptoms happen in the first two days after vaccination, with headache, fatigue and chills or shivers being the most common, according to the findings of the ongoing ZOE Covid Symptom Study app released on Thursday.

The researchers found that previous exposure to coronavirus makes it more likely that someone will experience systemic effects after vaccination.

The team analysed reports from nearly 40,000 people who were vaccinated in December - 23,308 people who had received just one dose, and 12,444 who had both.

The results showed that around four in ten people who received their first dose had at least one after effect in their arm, most commonly pain and swelling in the day or two after the jab.

The COVID Symptom Study was created by doctors and scientists working in partnership with ZOE - a health science company.

This research is led by Professor Tim Spector, Professor at King's College London.

"Our ongoing ZOE app data is so far showing reassuring signs of prevention after 12 days in 40,000 health care workers," Spector said in a tweet.

The Pfizer/BioNTech COVID-19 vaccine began its rollout in the UK on December 8. Since then, millions of people have been vaccinated, including the elderly, care workers and frontline NHS staff.

COVID-19 vaccines work by using a harmless version or component of the SARS-CoV-2 coronavirus to train the immune system, so when we encounter the virus for real we're able to fight it off.

The researchers were also able to compare the effects of vaccination in people who had already had Covid-19 with those who had not because many people have been continually using the ZOE COVID app throughout the pandemic.

The analysis showed that around a third of vaccinated people who had COVID-19 in the past reported at least one systemic effect within seven days from their vaccination, compared with just under one in five people who had not already had the virus.