KUALA LUMPUR, Dec 22 – Scientists at the United Kingdom’s Imperial College London estimate that a person with Omicron infects another three on average — with a reproduction rate above 3 — and infections do not seem less severe than Delta.
The study, reported ahead of peer review and updated on Monday, compared 11,329 people with confirmed or likely Omicron infections with 196,463 people infected with other variants.
Researchers found “no evidence of Omicron having lower severity than Delta, judged by either the proportion of people testing positive who report symptoms, or by the proportion of cases seeking hospital care after infection.” However, hospitalisation data remains very limited.
The results also suggest that the risk of reinfection with the Omicron variant is 5.4 times greater than that of the Delta variant. This implies that protection against reinfection by Omicron afforded by past infections may be as low as 19 per cent.
Researchers found a significantly increased risk of developing a symptomatic Omicron case compared to Delta for those who were two or more weeks past their second vaccine dose, and two or more weeks past their booster dose (for AstraZeneca and Pfizer vaccines).
Depending on the estimates used for vaccine effectiveness against symptomatic infection from the Delta variant, this translates into vaccine effectiveness estimates against symptomatic Omicron infection of between 0 per cent and 20 per cent after two doses, and between 55 per cent and 80 per cent after a booster dose.
Prof Neil Ferguson, who is lead author of the study, said the analysis 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,” Ferguson said.
Imperial College London’s Prof Azra Ghani said: “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.”