Early evidence has shown that two-doses of Covid vaccines are less effective against the Omicron variant, than against previous variants like Delta.
As a result, many countries have rolled out the booster doses. India will also begin the rollout of a third dose for health workers and elderly from January 10.
The study, led by the UK Health Security Agency (UKHSA), estimates the effectiveness of three doses of either of Pfizer, Moderna and AstraZeneca's Covid-19 vaccines against mild and severe disease in adults aged 65 years or older.
The study included individuals who reported symptoms and tested in community testing between November 27 and December 31, 2021. Vaccination rates in PCR positive cases were compared to vaccination rates in those who test negative.
The estimates suggest that effectiveness against symptomatic disease with the Omicron variant is significantly lower than compared to the Delta variant and wanes rapidly in those aged 65 years or older.
But 2 to 9 weeks post the third dose of a Covid vaccine was associated with an 89 per cent reduced risk of hospitalisation among symptomatic cases with the Omicron variant among those aged 65 years or older.
"The current data shows the booster dose is continuing to provide high levels of protection against severe disease, even for the most vulnerable older age groups," said Professor Wei Shen Lim, the Joint Committee on Vaccination and Immunisation's (JCVI) chair of Covid-19 immunisation, in a statement.
"The data is highly encouraging and emphasises the value of a booster jab," Lim added.
However, the UKHSA ruled out against the rollout of a second booster dose. In the wake of the highly infectious Omicron variant that is causing rising numbers of infections around the globe, Israel has started the fourth Covid shots, while some countries including Germany, France and the UK have been planning for it.
Priority should continue to be given to rolling out first booster doses to all age groups, the JCVI advised.