The Omicron variant of SARS-CoV-2 had a secondary attack rate of 25.1% in Norwegian households compared with 19.4% for the Delta variant.
The Omicron variant of SARS-CoV-2 was more likely to spread among household members than the Delta variant, a recent study confirmed, but the transmission rate of Omicron was smaller than expected.
The research letter, published in JAMA, used national data tracking all Norwegian residents from Dec. 1 to Jan. 8. and excluded single-person households or those with simultaneous index cases or cases in the three months leading up to the study period.
“Our main finding is that the Omicron variant of the SARS-CoV-2 virus, when introduced to Norwegian families, spread to 25% of the other family members,” Silje Bakken Jørgensen, MD, PhD, senior consultant in the Department of Infection Control and Preparedness at the Norwegian Institute of Public Health, told Contagion. “It is important to note that in the Norwegian population, 90% of all adults are vaccinated, and that the spread could be different in populations with a lower vaccination coverage amongst adults. There is no general recommendation for children under 12 years to be vaccinated in Norway.”
During the study period, Omicron increased from about 5% of cases isolated to 85%. Among 31,220 households with one index case there were 80,957 nonindex household members, 14% of whom were in households with an Omicron index case, 51% Delta and 35% nonclassified.
“Early studies on transmission of the Omicron variant indicated a larger transmission rate than what we found, and hence we were relieved to find a transmission rate that was only moderately larger than the transmission rates for the Delta variant,” Jørgensen said. “We found the transmission rate for Omicron in an almost fully vaccinated population to be equal to the transmission rate of the wild-type virus that was introduced in a population with no vaccination or other immunity.”
Secondary attack rates—defined as the percentage of nonindex household members with a positive test result within seven days after the index case—were 25.1% (95% CI, 24.4%-25.9%) in households with an Omicron infection, 19.4% (95% CI, 19.0%-19.8%) in households for Delta, and 17.9% (95% CI, 17.5%-18.4%) for nonclassified infections.
The odds ratio of nonindex household members testing positive was 1.52 (95% CI, 1.41-1.64) for Omicron compared with Delta. Odds ratios also were higher for men, unvaccinated household members, those older than 30 years and those tested at the end of the study period.
“It was interesting to see that the transmission rate was lower when the index case was vaccinated, which indicates that vaccination also could serve to limit the viral spread even if vaccination does not protect people from contracting mild disease,” Jørgensen said. “Our study was part of the research/knowledge base that made the Norwegian Institute of Public Health recommend a gradual re-opening of the society after strict measures had been introduced when the Omicron variant first emerged. The opening has not led to any increase in the number of patients admitted to intensive care units. Even though a large part of the population has been infected this spring, the infection curve has not peaked as abruptly as one could fear, but we have seen a more gradual rise and fall in COVID cases as our finding also would indicate.”
COVID-19 infections are currently at a low point, and some parts of the world are starting to see rising numbers. New research shows that the Omicron subvariant BA.2 is expected to overtake the original Omicron variant as the dominant strain, and it is more transmissible than the original Omicron variant. Investigators also are keeping an eye on the new “Deltacron” variant, a recombinant virus that combines the Omicron spike protein with the Delta virus and appears to be more transmissible.
“More research on how transmission rates change when or if the effect of vaccination is waning will be important for strategic planning regarding social infection control measures and health care preparedness,” Jørgensen said.