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* Omicron five times likelier to cause reinfections
* No sign milder than Delta, but some say too early to know
* Two-dose vaccines offer little or no Omicron protection
(Adds detail, reaction, quotes, context)
By Clara-Laeila Laudette
Dec 17 (Reuters) - The risk of reinfection with the Omicron
coronavirus variant is more than five times higher and it has
shown no sign of being milder than Delta, a study showed, as
cases soar across Europe and threaten year-end festivities.
The results of the study by Imperial College London were
based on UK Health Security Agency and National Health Service
data on people who tested positive for COVID-19 in a PCR test in
England between Nov. 29 and Dec. 11.
"We find no evidence (for both risk of hospitalisation
attendance and symptom status) of Omicron having different
severity from Delta," the study said, although it added that
data on hospitalisations remains very limited.
"Controlling for vaccine status, age, sex, ethnicity,
asymptomatic status, region and specimen date, Omicron was
associated with a 5.4-fold higher risk of reinfection compared
with Delta," the study, which was dated Dec. 16, added.
The protection afforded by past infection against
reinfection with Omicron may be as low as 19%, Imperial College
(ICL) said in a statement, noting that the study had not yet
been peer reviewed.
The researchers found a significantly increased risk of
developing a symptomatic Omicron case compared to Delta for
those who were two or more weeks past their second vaccine dose,
and two or more weeks past their booster dose.
The study involved AstraZeneca and Pfizer vaccines.
Depending on the estimates used for vaccine effectiveness
against symptomatic infection from the Delta variant, this
translates into vaccine effectiveness of between 0% and 20%
after two doses, and between 55% and 80% after a booster dose.
"This study provides further evidence of the very
substantial extent to which Omicron can evade prior immunity
given by both infection or vaccination," study lead Professor
Neil Ferguson said in ICL's statement.
"This level of immune evasion means that Omicron poses a
major, imminent threat to public health."
TOO EARLY?
But Dr Clive Dix, former Chair of the UK Vaccine Taskforce,
said it was important not to overinterpret the data.
"The conclusions made are based on making assumptions about
Omicron where we still don't have sufficient data," Dr Dix said.
"For example, we have no data on the cellular immune response
which is now probably driving effectiveness of vaccines."
"This is a crucial missing assumption in the modelling."
Some of the conclusions are different to the data emerging
from South Africa, where vaccines are holding up well against
severe disease and death at present, he said.
"There is a huge amount of uncertainty in these modelled
estimates and we can only be confident about the impact of
boosters against Omicron when we have another month of
real-world data on hospitalisation ICU numbers and deaths," he
said.
An earlier study by Britain's SIREN looking at reinfection
risk in health workers, which was carried out before Omicron
emerged, found that a first coronavirus infection offered 85%
protection from a second for the following six months.
The data analysed by Imperial College was based on 333,000
cases, including 122,062 of Delta and 1,846 which were confirmed
as the Omicron coronavirus variant through genome sequencing.
Imperial College's Professor Azra Ghani, who co-led the
study, described it as "essential for modelling the likely
future trajectory of the Omicron wave and the potential impact
of vaccination and other public health interventions."
The new findings could accelerate the imposition of tighter
restrictions across a number of European countries in a bid to
stem the new variant's spread.
(Reporting by Clara-Laeila Laudette and Jo Mason; Editing by
Alexander Smith)
