Humans infected with the Omicron variant of coronavirus SARS-CoV-2 are almost 50% more likely to infect those they live with than individuals infected with the Delta variant, a detailed analysis shows from England1.
Since it was first discovered in South Africa in late 2021, Omicron has caused a wave of infections and deaths in countries across the globe, including places like Hong Kong, which have been largely spared by SARS-CoV-2 until now. Data have increased on Omicron’s ability to evade vaccines, but a rigorous analysis published as a pre-print on 17 February looks at how quickly it spread – even in vaccinated people – compared to the previous variant. The study has not yet been peer reviewed.
In the UK, Omicron was first discovered in mid-November 2021 and quickly overtook Delta as the dominant variant. Its rapid spread was likely driven by its ability to evade immunity generated by vaccines, says Marm Kilpatrick, an infectious disease researcher at the University of California, Santa Cruz. These types of immunosuppressive variants will continue to be selected for, he says – “we can almost expect them to occur”.
Researchers at the UK Health Security Agency analyzed data on 51,281 people who tested positive for Omicron or Delta between 5 and 11 December 2021 and their 151,592 close contacts.
The researchers found that while people infected with Delta spread the infection to about 11% of their household members, those who had Omicron spread it to almost 16% (see ‘Comparison of variants’). This means that there is a 48% increased risk of transmitting the virus when infected with Omicron than with Delta.
This difference is even more striking outside the home – people infected with Delta spread the infection to about 4% of the people they came in contact with outside their home, while those with Omicron passed it on to 8% of the population, so the risk was more than doubled. Omicron’s transmission advantage is more pronounced outside the household, because even with fewer opportunities for repeated and prolonged exposure to the virus, the risk of catching it is more than double compared to Delta, Kilpatrick says.
The researchers also assessed how vaccination affected people’s chances of catching the two variants, and found that unvaccinated household members were 23% more likely to be infected with Omicron than with Delta.
But that gap widened in people who have been vaccinated, revealing Omicron’s advantage in avoiding vaccine-triggered immunity. Household members who had received three doses of a SARS-CoV-2 vaccine were more than twice as likely to become infected with Omicron than with Delta (see ‘Vaccine effect’).
If someone had received three doses of vaccine, they were also about twice as likely to pass on Omicron to another household member than they were Delta (see ‘Transmission Blocking’).
Three is better than two
The data also show that a third dose of vaccine had a marginal beneficial effect on the transmission of both variants compared to only two doses, although this effect was greater for Delta than for Omicron. Following a third dose, household members were approximately 32% less likely to become infected with Delta and 12% less likely to become infected with Omicron than if they had received only two doses. People who had received a booster shot were 38% less likely to spread a Delta infection to their household members and 22% less likely to pass on an Omicron infection than those who had received only two shots.
Although this study suggests that existing SARS-CoV-2 vaccines offer limited protection against infection with Omicron, other research has shown that they are effective in preventing serious illness caused by both variants, says Leo Poon, a virologist at the University. of Hong Kong. But he points out that the work was done at a time when the BA.1 sub-variant of Omicron was circulating, and that now that another sub-variant, BA.2, is rapidly on the way. wins steam, this variant must also be investigated. Nevertheless, vaccination is still one of the most effective measures to “protect ourselves” from serious illness, hospitalization and death, he says.