Singapore reported its first local Omicron case on Thursday - a front-line worker at Changi Airport who was infected despite already receiving the booster shot of the Covid-19 vaccine.
The Straits Times looks at what is known and what remains unknown about this new variant of the coronavirus that causes Covid-19, and what this could mean for Singapore.
Q When did Omicron emerge?
A A new variant of the coronavirus, known as B.1.1.529, was reported to the World Health Organisation (WHO) on Nov 24. Two days later, the WHO named it Omicron and classified it as a variant of concern.
The variant had been first detected in Botswana on Nov 11 and in South Africa on Nov 14. It has been picked up in more than 30 countries since.
A variant results when a virus mutates spontaneously as it replicates and spreads.
Omicron has been found to be very different from previous variants due to the numerous mutations it has undergone. Its spike protein alone has been found to have 32 mutations.
Q Are current Covid-19 vaccines still effective?
A Vaccines train our bodies to attack the spike protein on the surface of the virus. Due to the location of Omicron's numerous mutations on the crown-like spike protein, the virus is likely to evade vaccines to some extent.
Some of the spike mutations are in the same locations seen in other variants that spread quickly.
A preliminary study by South African scientists published on Dec 2 suggested that Omicron is three times more likely to cause reinfections compared with the Delta or Beta strain. The findings were based on data collected by the country's health system. The paper has not yet been peer-reviewed.
In a Dec 8 update, BioNTech and Pfizer said that a three-shot course of their Covid-19 vaccine was able to neutralise the new Omicron variant in a laboratory test, and they could deliver an Omicron-based vaccine in March next year if needed.
The vaccine manufacturers said that two vaccine doses resulted in significantly lower neutralising antibodies, but a third dose of their vaccine increased the neutralising antibodies 25 times.
There is also still no sign that Omicron will cause a significant share of vaccinated people to get severe versions of Covid-19. However, it could prove to be a concern if seemingly mild infections turn out to be deadly for vulnerable groups, such as the elderly.
Q Does this mean booster shots are needed every few months if more variants of the virus surface?
A Boosters reduce the risk of infection, making up for waning immunity, but it is unclear how long this lasts, said Associate Professor Alex Cook, vice-dean of research at the National University of Singapore's Saw Swee Hock School of Public Health.
"It could be that boosters are long-lasting and the third dose is all we need for lasting protection against infection, but it could also be that immunity wanes again, just like with the first two shots, which is compromised by the emergence of new variants," Prof Cook said.
"If the latter is true, after we get past the pandemic phase, the health system may be able to cope even if only the most vulnerable take occasional booster doses, just as it does with influenza."
Q Does Omicron spread faster than earlier variants?
A Data out of South Africa suggests that Omicron is spreading more quickly than Delta, and there is some preliminary evidence that it is more infectious than Delta.
A question that has yet to be answered is why Omicron is spreading rapidly, and if the spread is a result of immune evasion.
There is no evidence so far that standard prevention strategies, such as vaccination, masking, distancing, ventilation and hand washing are not effective in reducing the risk of infection or transmission, Johns Hopkins University reported.
Q Does Omicron cause more severe illness?
A There is very limited data on this as the variant was just discovered, but so far, it does not appear so.
Some scientists think it is still too early to answer this as severe illness often takes a week or more to develop.
Initial studies of Omicron patients have also come disproportionately from South Africa, where the population is younger and many people were previously infected with Delta. These groups are less likely to fall very ill.
Q Does Omicron spell tighter community measures for Singapore?
A Unless borders were completely shut, it was always a matter of time before Omicron arrived in Singapore and started circulating in the community, said Professor Teo Yik Ying, dean of the Saw Swee Hock School of Public Health.
The recent announcement by WHO that vaccinations are still effective in protecting against severe disease and death means that Singapore's endemic Covid-19 strategy of relying on vaccination, boosters and natural infection is still relevant.
"What is important is to continue to monitor the data arising from Omicron infections, to be sure that there is no increased risk of symptomatic infections and severe disease for people who are fully vaccinated. Hence, there is presently no need to retighten any of the measures," Prof Teo said.
Dr Asok Kurup, an infectious disease physician at Mount Elizabeth Hospital, said: "We are just seeing the first few cases of Omicron, and it's a good thing that we are picking up these cases in the first place."
The detection of Omicron cases shows that Singapore has a surveillance system in place, and the authorities can mitigate the onward spread and transmission as much as possible, he added.
Prof Cook said: "Given the very high vaccine coverage Singapore has, it is not clear, to me at least, whether the measures to prevent Omicron transmission - which would have to be very onerous to have any chance of success - are justified... Instead, I would advocate for a more aggressive roll-out of booster shots, to give further protection against severe disease, and letting the virus circulate."
Q Do current antigen rapid tests (ARTs) still work?
A So far, the analysis has shown that ARTs remain effective in detecting Omicron. Only some polymerase chain reaction (PCR) tests can distinguish an Omicron from a Delta infection, but PCR tests lack the ease of use that ARTs have.
Q What does Omicron mean in the long term?
A Omicron further cements the idea that the Sars-CoV-2 virus that causes Covid-19 is highly adaptable, and may be difficult to eradicate entirely. This means predictions about the pandemic are difficult to make.
Even if Omicron does not result in more severe infections for most people, having more cases would naturally result in more hospitalisations and deaths.
With large numbers of unvaccinated people around the world, the virus will keep spreading and mutating - and new variants that emerge may or may not be more transmissible and/or deadly.
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