A complete course of coronavirus immunization protects against Omicron infection in just 20% of patients.

The Omicron mutation is immune to past infection or two vaccine doses, according to the latest Imperial modeling.

The Imperial College London COVID-19 reaction team has released a new analysis (Report 49) that estimates the probability of reinfection with the Omicron strain is 5.4 times higher than that of the Delta variant. This means that past infection protection against Omicron reinfection may be as low as 19%.

The current study from England attempts to predict how the Omicron variant would spread and if it will be immune to the virus. All PCR-confirmed SARS-CoV-2 cases in England who took a COVID test between November 29th and December 11th 2021 were included in the study.

The study included participants with Omicron infection who had an S gene target failure (SGTF) as well as people with Omicron infection who had genotype data. The SGTF analysis identified 196,463 patients without S gene target failure (likely to be infected with another variant) and 11,329 cases with it (likely to be infected with Omicron), 122,063 Delta and 1,846 Omicron cases.

Spread of Omicron

First, the report compares Omicron positive cases versus non-Omicron (mainly Delta) patients. On the basis of S-gene Target Failure and genotyping data, the fraction of Omicron among all COVID cases doubled every 2 days till December 11th. Based on these findings, they estimate Omicron’s reproduction number (R) to be over 3.

Omicron infection rates vary greatly by age, area, and ethnicity, with 18–29-year-olds, residents of the London region and those of African ancestry having significantly greater rates of infection than Delta. London outperforms the rest of England in Omicron frequency.

Omicron transmission is still not evenly distributed. The researchers warn that due to Omicron’s immunological evasion, its age distribution may continue to diverge from Delta’s.

The researchers found no evidence that Omicron is less severe than Delta in terms of the number of people who test positive and report symptoms, or the proportion of cases who require hospital treatment following infection. However, hospitalization data is still scarce.

Re-infection rates

Scientists employed genotype data to examine Omicron’s influence on reinfection rates since, previous to Omicron, reinfection was associated with negative S gene Target Failure data, possibly due to reduced viral loads causing random PCR target failure.

Compared to Delta, Omicron was linked with a 5.40 (95% CI: 4.38-6.63) fold higher risk of reinfection after controlling for vaccine status, age, sex, ethnicity, asymptomatic status, area, and specimen data. Before Omicron, the UK’s “SIREN” research of COVID infection in healthcare workers estimated that prior infection provided 85% protection against subsequent infection over 6 months. The current study’s predicted probability of reinfection is 19% (95% CI: 0-27%).

Vaccine efficacy against Omicron

Those two weeks or more beyond their second immunization dose and two weeks past their booster dosage had a significantly greater probability of developing Omicron symptoms compared to Delta (for AstraZeneca and Pfizer vaccines). 

The vaccine’s effectiveness against symptomatic Omicron infection can range from 0% to 20% after two doses, and from 55% to 80% after a booster dose. This is based on the estimates used for vaccine effectiveness against the Delta variant. Comparable estimates with larger uncertainty using genotype data.

“This study provides further evidence of the very substantial extent to which Omicron can evade prior immunity given by both infection or vaccination,” stated Imperial College London’s Prof Neil Ferguson, adding, “This level of immune evasion means that Omicron poses a major, imminent threat to public health.”

“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,” according to Imperial College London’s Azra Ghani.

The work, which has not yet been peer-reviewed, is published in the most recent report from the WHO Collaborating Centre for Infectious Disease Modelling at the MRC Centre for Global Infectious Disease Analysis, Jameel Institute, Imperial College London.

Source: Report

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