After analysis of the new Coronavirus strain, Omicron, scientists have concluded that those who have received both vaccines could still contract the disease.
Despite this, a third booster prevents approximately 75% of people from getting Covid symptoms.
The UK Health Secretary previously said that receiving both vaccines will offer good protection against severe Covid cases which result in-hospital treatment.
The analysis is based on limited data but showed a dramatic drop in effectiveness for the Oxford-AstraZeneca vaccine and a significant drop off for two doses of the Pfizer vaccine.
Currently, around 22 million people have received the booster jab and experts are optimistic that even those who do contract the virus will unlikely suffer severe symptoms resulting in fewer Covid hospital numbers.
It is estimated that the number of Omicron cases is doubling every two-to-three days, with scientists estimating that more than half of all cases in the country will be Omicron by mid-December, resulting in an increased number of cases a day.
Dr Mary Ramsay, the head of immunisation at the UKHSA said:
"These early estimates should be treated with caution but they indicate that a few months after the second jab, there is a greater risk of catching the Omicron variant compared to Delta strain."
"We expect the vaccines to show higher protection against the serious complications of COVID-19, so if you haven't yet had your first two doses please book an appointment straight away."
A further 58,194 Covid cases have been reported in the UK, according to the latest government statistics. This is mostly still the Delta variant, but is the highest figure since 9 January this year.