Two years into the pandemic, the only thing we are certain of is the ever-changing, ever-mutating nature of the Sars-CoV-2 virus that causes Covid-19. Since the start of the outbreak in December 2019, the virus has spawned a series of variants named after Greek alphabets Alpha, Beta, Gamma, Delta and now, Omicron.

The World Health Organisation (WHO), while labelling the new variant as Omicron, classified it as "highly transmissible" in November 2021. Since then, the variant has wreaked havoc in several countries, driving their number of cases to record highs.

Even as case numbers rise across the world — the United States has recently been averaging 800,000 new infections every day — health experts have predicted that the Omicron death toll may remain lower than the devastation caused in previous waves, particularly the one driven by Delta.

The discovery of Omicron

The new variant of the coronavirus was first detected in South Africa at the end of November last year, with scientists saying it was behind the spike in infections in the country. As with other variants, some infected people display no symptoms, South African experts had said at the time.

Shortly after, the WHO named the new variant Omicron and classified it as "highly transmissible".

The variant has about 50 mutations not seen in combination before, including more than 30 mutations on the spike protein the coronavirus uses to attach to human cells. Vaccines in current use target that spike protein, according to a Reuters report. This means protection provided by Covid-19 vaccines may be less effective on Omicron.

How transmissible is Omicron?

One of the earliest studies on the transmissibility of Omicron, conducted by researchers in the United Kingdom, revealed that the new variant was two to three times more likely to infect people, specifically in household settings, as compared to Delta.

Associate Professor of Epidemiology at Harvard's T. H. Chan School of Public Health, Bill Hanage, said Omicron was expected to show a tendency to clustered transmission in large gatherings.

A more recent study conducted in France suggested that Omicron may be 105 per cent more transmissible than Delta. The study, which analysed over 131,000 samples but is yet to be peer-reviewed, noted that even if the severity of Omicron was reduced compared to Delta, the higher reproduction number — the number of people in a population who can be infected by an individual at any specific time — meant that French hospitals may see higher activity, if not overloading.

Another factor that sets Omicron apart is the increased risk of reinfection. A study by Imperial College London showed that Omicron was 5.4 times more likely to reinfect than Delta.

In a recent press conference, WHO's technical lead on Covid-19, Maria Van Kerkhove, said Omicron was "efficiently transmitting" and being detected in all areas with good gene sequencing options.

Explaining why Omicron has led to a massive jump in case numbers, Kerkhove said the virus was able to adhere to human cells more easily because of its mutations, cause reinfection, and replicate in the upper respiratory tract, which she said, set it apart from other variants like Delta.

"This combination of factors allowed the virus to spread more easily. With Omicron, what we're learning is that your risk of getting severe disease is less than it is for Delta," she said. The infectious diseases expert cautioned, however, that the global health body was still seeing a rise in the number of hospitalisations and people who needed clinical care which would put pressure on countries' health systems.

What about severity?

While the number of infections is significantly higher, the rates of deaths, and even hospitalisations, are much lower when compared to Delta.

This is backed by several studies, including one conducted by South African scientists that found people diagnosed with Omicron over a period of two months were 80pc less likely to require hospitalisation than people infected with other variants. The scientists, however, said that a high level of immunity was partly behind the reduced need for hospitalisation in the country.

One of the study's authors, Professor Cheryl Cohen of the National Institute for Communicable Diseases, said, "Compellingly, together our data really suggest a positive story of a reduced severity of Omicron compared to other variants."

Separately, the UK Health Security Agency's research found the risk of hospitalisation after being infected with the Omicron variant was half as that with Delta. In addition, their study said that the risk of hospitalisation was reduced by 81pc in people who had received two and three doses of a Covid-19 vaccine compared to unvaccinated people.

In Scotland, researchers at the University of Edinburgh found that the number of Omicron patients who needed to be hospitalised was 68pc less compared to Delta. However, their study also underscored the increased risk of reinfection — 7.6pc of the samples analysed were reinfections compared to 0.7pc with Delta.

Yet another study, conducted by Imperial College London, suggested that the risk of hospitalisation for Omicron patients was 40 to 45pc lower compared to the Delta variant.

Earlier this month, Portuguese Prime Minister Antonio Costa said it was "evident that Omicron is less severe" as he announced the easing of curbs despite a record rise in cases.

Closer to home, Indian health officials said that only five to 10pc of patients in the current wave driven by Omicron have required hospitalisation in contrast to 20 to 23pc in the previous wave driven by Delta.

However, hospitalisations have not remained low in all countries. There were 132,646 people hospitalised with Covid on Jan 10 in the United States, surpassing the record of 132,051 set in January 2021.

Elsewhere, hospitalisations in France have seen their biggest increase since April 2021 but remain below the peaks set at the end of 2020. The country's health minister said Omicron leads to less severe complications but hospitalisations were still increasing because it was highly infectious.

Read: N95, surgical or cloth — Is one face mask better than the other for optimum protection against Omicron?

Omicron affects airways, not lungs

Experts say one reason behind the reduced severity may be that Omicron multiplies faster in the airways and lower in the lungs.

According to researchers at the University of Hong Kong, "at 24 hours after infection, the Omicron variant replicated around 70 times higher than the Delta variant and the original Sars-CoV-2 virus. In contrast, the Omicron variant replicated less efficiently (more than 10 times lower) in the human lung tissue than the original Sars-CoV-2 virus, which may suggest lower severity of disease."

However, it cautioned that the overall threat from the variant was "very significant" because of its increased transmissibility.

According to a report by The Guardian, six separate studies found that Omicron does not cause as severe damage to the lungs as Delta and other variants.

Last week, WHO Incident Manager Abdi Mahamud, too, said more evidence was emerging that Omicron caused less severe illness because it affected the upper respiratory tract.

"We are seeing more and more studies pointing out that Omicron is infecting the upper part of the body unlike the other [variants] that could cause severe pneumonia," he was quoted as saying by Reuters.

Do existing vaccines offer protection against Omicron?

While protection against Omicron may wane a few months after receiving two shots of a vaccine, multiple studies showed that getting a booster dose produces antibodies capable of fighting the new variant.

Early findings from a real-world analysis suggested that the risk of catching Omicron was "significantly reduced following a booster vaccine", Head of Immunisation at the UK Health Security Agency Dr Mary Ramsay said.

"Two doses of AstraZeneca or Pfizer-BioNTech vaccines provided much lower levels of protection against symptomatic infection compared with what they provide against Delta.

"However, when boosted with a dose of Pfizer vaccine, there was around 70pc protection against symptomatic infection for people who initially received AstraZeneca, and around 75pc protection for those who received Pfizer," according to a Reuters report.

Studies conducted later also pointed to the same conclusion — booster doses offer protection against Omicron.

Danish researchers found that vaccine effectiveness for those who had received a shot of Pfizer or Moderna was re-established when they were given a booster dose of the Pfizer vaccine.

Research carried out by the pharmaceutical companies themselves has also shown encouraging results. Pfizer said a three-course shot of its vaccine was able to neutralise the Omicron variant in a lab test. Its CEO also said that the company is working on an Omicron-specific vaccine that will be ready by March.

Moderna said a booster dose of its vaccine appeared to be effective against Omicron, adding that it "boosts neutralising antibody levels 37-fold higher than pre-boost levels".

A study by the University of Oxford, while supporting a third dose of AstraZeneca's vaccine, concluded it had boosted neutralisation levels.

However, research on more doses of Chinese vaccines — Sinopharm and SinoVac — has not been as encouraging.

A joint study conducted by the University of Hong Kong and the Chinese University of Hong Kong showed that "third dose of [SinoVac] given to those who received two previous doses of [SinoVac] does not provide adequate levels of protective antibody."

Researchers from Shanghai Jiao Tong University and a Shanghai-based lab specialising in respiratory infectious diseases found that Sinopharm's vaccine had "significantly lower" neutralising activity against the Omicron variant although they added the vaccine's efficacy against Omicron remained unclear.

A later study suggested that a protein-based vaccine produced by Sinopharm "significantly improved the immune responses against various Sars-CoV-2 strains, including Omicron" when administered after two doses of the original vaccine.

Other treatments are also available this time around. The US Food and Drug Administration (FDA) has approved two anti-viral pills — from Pfizer and Merck — that could be an important tool against Omicron.

Both companies said they expect their pills to be effective against the new variant.