Vaccine effectiveness against the Omicron variant is "significantly lower" than with the Delta variant but a booster dose raises protection against mild cases, a new study has found.
Research released on Friday by the UK Health Security Agency found that a full two-dose vaccination course was less effective against symptomatic disease with Omicron than with the original strain of COVID-19 or the Delta variant.
It found however that "moderate to high vaccine effectiveness against mild infection of 70-75% was seen in the early period after a booster dose."
Omicron was designated a variant of concern by the World Heath Organisation (WHO) in late November over concerns its high number mutations could make it more transmissible and more resistant to current treatment, including vaccines.
The variant, which was first detected by South African laboratories, has now spread to nearly 60 countries worldwide. Almost all of the nearly 2,200 confirmed cases of Omicron have however so far been either asymptomatic or mild.
WHO chief Dr. Tedros Adhanom Ghebreyesus emphasised during his regular COVID-19 briefing on Thursday that "certain features of Omicron, including its global spread and large number of mutations, suggest it could have a major impact on the course of the pandemic."
"Exactly what that impact will be is still difficult to know," he noted however, stressing that "new data are emerging every day, but scientists need time to complete studies and interpret the results."
"We call on all countries to increase surveillance, testing and sequencing," he also said.
The latest British study, which has not yet been peer-reviewed, was unable to determine protection against severe forms of disease due to the small number of Omicron cases so far "and the natural lag between infection and more severe outcomes."
"Previous UK experience with the Delta variant suggested that protection against hospitalisation after 2 doses of vaccine was relatively well maintained," it said.
The study also noted that booster doses of the Pfizer/BioNTech vaccine "provide a significant increase in protection against mild disease and are likely to offer even greater levels of protection against severe disease."
"As such our findings support maximising coverage with third doses of vaccine in highly vaccinated populations such as the UK," it concluded.
According to the European Centre for Disease Prevention and Control (ECDC) about 500 confirmed cases of Omicron have been reported by 21 countries in the European Union and European Economic Area.
The EU agency highlighted that "although cases reported initially were linked to travel, an increasing number of cases are now being reported as acquired within the EU/EEA, including as part of clusters and outbreaks, with cases also being detected in representative surveillance systems."
"There are indications that undetected community transmission could be ongoing in EU/EEA Member States," it said.
Just under 67% of the EU/EEA's population is now fully vaccinated but there are vast disparities between member states.
Denmark and Portugal have fully vaccinated more than 80% of their total population, while most Western or Nordic countries have rates over 70%. Uptake of the vaccine has been much slower in eastern Europe with only 26.2% and 39% of the Bulgarians and Romanians now fully jabbed.