Omicron is by far the most contagious variant of SARS-CoV-2 to date. It has already spread to 77 countries and is currently causing massive outbreaks in South Africa, United Kingdom, and Denmark. Although the levels of hospitalization and deaths have remained relatively low in South Africa, where the variant is dominant since almost three weeks, experts are not yet sure whether Omicron causes milder disease than previous strains.
In fact, according to a new study led by Imperial College London, Omicron does not seem to have lower severity than Delta, judged by either the proportion of symptomatic infections or hospitalizations in the United Kingdom, where Omicron cases are expected to peak by the end of the year with daily numbers rising up to one million.
“We find no evidence (for both risk of hospitalization attendance and symptom status) of Omicron having different severity from Delta,” the study authors wrote.
By analyzing a cohort of over 333,000 people who tested positive for the coronavirus between November 29th and December 11th, scientists estimated that the risk of reinfection with Omicron is 5.4 greater than that of the Delta variant. This implies that protection against reinfection after a previous infection may be as low as 19 percent.
The researchers also found that vaccine effectiveness against Omicron was significantly lower than in the case of other variants (between 0 percent and 20 percent after two doses of Pfizer or Astra Zeneca, and between 55 percent and 80 percent after a booster dose). However, vaccines still appear to offer a significant degree of protection against severe illness and death.
“This study provides further evidence of the very substantial extent to which Omicron can evade prior immunity given by both infection or vaccination. This level of immune evasion means that Omicron poses a major, imminent threat to public health,” said study lead author Neil Ferguson, a professor of Epidemiology at Imperial College London.
“Quantifying reinfection risk and vaccine effectiveness against Omicron is essential for modelling the likely future trajectory of the Omicron wave and the potential impact of vaccination and other public health interventions,” added study co-author Azra Ghani, the Chair in Infectious Disease Epidemiology at the same university.
Although this study appears rather pessimistic, it is still too early to clearly assess the virulence of Omicron. According to Dr. Clive Dix, former Chair of the UK Vaccine Taskforce, at least another month is needed to clarify the levels of vaccine efficacy.
“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 hospitalization, ICU numbers, and deaths,” he said.