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 lower than expected.
The research letter, published in JAMAused national data on all Norwegian residents from December 1 to January 8 and excluded single-person households or those with index cases or concurrent cases in the three months prior to the study period.
“Our main finding is that the Omicron variant of the SARS-CoV-2 virus, when introduced into Norwegian families, spread to 25% of 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, said Contagion. “It is important to note that in the Norwegian population, 90% of all adults are vaccinated, and the spread could be different in populations with lower vaccination coverage among adults. There is no general recommendation for children under 12 to be vaccinated in Norway.
Over the study period, Omicron increased from approximately 5% of isolated cases to 85%. Among 31,220 households with an index case, there were 80,957 non-index household members, of which 14% were in households with an Omicron index case, 51% Delta and 35% unclassified.
“Early studies of the transmission of the Omicron variant indicated a higher transmission rate than what we found, and so we were relieved to find a transmission rate that was only moderately higher than the rates of transmission of the Delta variant,” said Jørgensen. “We found that the transmission rate of Omicron in a nearly fully vaccinated population was equal to the transmission rate of the wild-type virus that was introduced into a population with no vaccination or other immunity.”
Secondary attack rates – defined as the percentage of unlisted household members with a positive test result within seven days of the index case – were 25.1% (95% CI, 24.4% at 25 .9%) in households with Omicron infection, 19.4% (95% CI, 19.0%-19.8%) in households with Delta, and 17.9% (95% CI, 17. 5%-18.4%) for unclassified infections.
The odds ratio of non-indexed positive household members was 1.52 (95% CI, 1.41-1.64) for Omicron compared to Delta. Odds ratios were also higher for men, unvaccinated household members, people over the age of 30, and those tested at the end of the study period.
“It was interesting to see that the rate of transmission was lower when the index case was vaccinated, indicating that vaccination could also serve to limit viral spread even if vaccination does not protect people against mild illnesses,” Jorgensen said. “Our study was part of the research/knowledge base that prompted the Norwegian Institute of Public Health to recommend a gradual reopening of society after the introduction of strict measures when the Omicron variant first emerged. 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 might fear, but we have seen a more gradual increase and decrease in COVID cases, as our finding also indicates.
COVID-19 infections are currently at a low level and some parts of the world are starting to see rising numbers. New research shows that the BA.2 subvariant of Omicron is expected to overtake the original Omicron variant as the dominant strain, and is more transmissible than the original Omicron variant. Investigators are also keeping an eye out for the new “Deltacron” variant, a recombinant virus that combines the spike protein Omicron with the Delta virus and appears to be more transmissible.
“More research on how transmission rates change when or if the effect of vaccination decreases will be important for strategic planning regarding social infection control measures and health care preparedness,” Jørgensen said.