From infection rates to vaccine efficacy: how Omicron differs from Delta

The detection of the Omicron variant of Covid-19 in Botswana and South Africa in November 2021 represented another milestone in the pandemic. Delta, which was first detected in India the previous year, had become the dominant form of Sars-CoV-2 in many countries because of its ease of spread. But Omicron was able to pass between people even more readily, and this meant that it took over as the most common variant and caused huge surges in cases in many parts of the world. However, as was suspected by a doctor in South Africa who dealt with some of the first Omicron cases the new variant typically causes less severe disease. This has been confirmed by subsequent studies. So while case numbers have rocketed, death rates often have not. According to data from a study in California cited by the US Centres for Disease Control and Prevention, Omicron is associated with a 53 per cent lower risk of hospital admission compared to Delta. The research from January also found that the chance of being admitted to an intensive care unit was reduced by 74 per cent with Omicron, while the risk of death decreased even more steeply — by 91 per cent. In some nations, this has given officials confidence to lift lockdown measures, even in the face of high case numbers. At the beginning of this month, Denmark became the first European Union country to lift all Covid-19 restrictions — despite having one of the highest infection rates in the world at the time. Prof Eskild Petersen, of Aarhus University Hospital in Denmark, who is chairman of the European Society of Clinical Microbiology and Infectious Diseases, said a combination of high vaccination levels and Omicron being “milder” made this possible. “Compared to a year ago, both the hospitalisation rate and the death rate is reduced by 90 per cent,” he said of the situation in Denmark. “It’s milder, Omicron, and we have the vaccination. What is the vaccination and what is Omicron, I don’t know.” While Omicron is milder, health officials have been keen for the public not to become complacent and say there are still significant risks. The CDC, for example, warns that people may still have severe disease, require hospital care and die. “Even if only a small percentage of people with Omicron infection need hospitalisation, the large volume of cases could overwhelm the healthcare system, which is why it’s important to take steps to protect yourself,” the organisation said. While the Omicron variant has been associated with milder infections, it does not mean that the coronavirus is no more of a concern than, say, influenza, said Prof John Oxford, emeritus professor of virology at Queen Mary University of London and co-author of the textbook Human Virology . “It’s kind of a bigger threat than flu,” he said. “Omicron is highly infectious, [with] massive amounts of virus in the upper airways. We still don’t quite understand its killing power. “With influenza, many people die from subsequent bacterial infections, but with these, antibiotics are a protective backstop.” With Covid-19, he said, there is no such backstop. “It’s an immune reaction of their own body, so antibiotics aren’t much use,” he said. “So we’re going to have to look more deeply into how they are dying. What’s the immune reaction that’s killing these people?” Omicron is characterised by about 50 mutations, 36 of which are on the spike protein, the section of the coronavirus that attaches to human cells. This is also the section of the virus detected by the antibodies produced by vaccination. The European Medicines Agency (EMA) reports that vaccines are less effective at preventing symptomatic disease with Omicron than with other variants. Before Omicron emerged and Delta was the predominant variant, two doses of the Pfizer-BioNTech vaccine had a 93 per cent effectiveness at preventing the need for hospital admission, according to a South African study. The same vaccine was just 70 per cent effective at preventing the need for hospital admission with Omicron, a fall of 23 per cent efficacy. However, the UK Health Security Agency said that after a booster or third dose, vaccination’s effectiveness at preventing the need for hospital admission increased to 88 per cent. Data from the Washington State Department of Health in the US indicates that Omicron is less likely than other key variants to produce a symptomatic infection. Omicron also behaves differently with respect to Covid-19 tests. South African research has found that mouth swabs are better than nasal swabs at detecting Omicron, whereas with Delta, nasal swabs are more effective. Rapid tests are thought to be less likely to detect Omicron than earlier variants. A sub-variant of Omicron called BA.2 has in some nations outcompeted the original form of Omicron, which is referred to as BA.1. In countries as far apart as India and Denmark, BA.2, which has 20 mutations differentiating it from BA.1, has already become dominant. It is suspected to be transmitted more easily than the original Omicron. The UK HSA said in a technical briefing at the end of January that 13.4 per cent of contacts of people with BA.2 were found to have tested positive, compared to 10.3 per cent with BA.1. However, the UK HSA reported that “a preliminary assessment” did not find that vaccines were less effective at preventing BA.2 from causing symptomatic disease, compared to BA.1. BA.2 may be more difficult for PCR testis to detect, thanks to the presence of what has been dubbed a “stealth deletion”. Although increases in case rates with Omicron have typically not resulted in the steep increases in deaths seen previously, some specialists remain cautious. Dr Bharat Pankhania, a senior consultant in communicable disease control and senior clinical lecturer at the University of Exeter in the UK, said it was only possible for authorities to be more relaxed about lockdown measures now because populations have been heavily vaccinated. “It’s the immunisation that puts us in a safer place,” he said. “It’s not that suddenly Omicron is innocuous. The successful uncoupling [of case rates and death rates] is that it’s not death-causing if you’re fully immunised. “Without a doubt Omicron is a nasty piece of work if you’re not immunised. It’s the vaccination that’s preventing people from becoming seriously ill, rather than that we’ve got a less nasty virus in circulation.”

From infection rates to vaccine efficacy: how Omicron differs from Delta

Despite worldwide surge, latest Covid-19 mutation appears less fatal, but experts 'don’t quite understand its killing power'