The US Centers for Disease Control and Prevention has investigated a cluster of Omicron infections in Nebraska state and found that the variant causes a faster onset of infection symptoms. But, people who get reinfected with the highly transmissible variant, may experience fewer symptoms than they did during their initial bout with the virus.

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The CDC, in its weekly latest Morbidity and Mortality Weekly Report published on 28 December, reported the case of six people in a single household with confirmed cases of Omicron.

Of the six, an unvaccinated man aged 48 years (the index patient) had recently returned from Nigeria. Before his return trip to the US, he tested negative for COVID on November 21, but tested positive on November 26. The man was first infected with COVID in last year.

After returning from Nigeria, he had unmasked close contact with five household contacts. One household contact was fully vaccinated and had previous symptomatic COVID in 2020, three were unvaccinated and were infected with COVID last year, and one remained unvaccinated and had mild upper respiratory symptoms last year, but tested negative for COVID.

While the index patient tested positive on November 26, all six household members aged 11-48 years experienced symptom onset during November 24-26. The median interval between earliest possible exposure to the index patient and symptom onset was 73 hours, the CDC said.

Importantly, the CDC found that the five patients - all cases of reinfection - described the symptoms and severity of their recent COVID infection as being similar to or milder than those during their first infection.

"The five reinfected patients experienced fewer current symptoms, including loss of taste (none), loss of smell (none), and subjective fever (two), compared with symptoms reported during their first infections (four, four, and four, respectively). The unvaccinated patient without a previous COVID-19 diagnosis experienced cough, joint pain, congestion, fever, and chills," the CDC said.

However, none required hospitalisation for either their first or second infections. "Observations from this investigation suggest a shorter incubation period," wrote Lauren Jansen, from Nebraska Department of Health and Human Services, in the CDC report.

It also shows "A clinical syndrome similar to or milder than that associated with previously described variants in persons who have been vaccinated or previously infected, and add to existing evidence suggesting an increased potential for reinfection", she added.

While infection with Delta variant has upto 4 days of incubation period variant, the median incubation period observed in Nebraska cluster was approximately 3 days, the report said.

So far infection from Omicron has been reported to be mild among vaccinated patients. However, it is unknown whether the mild clinical syndromes or differing symptom descriptions are a result of existing immunity or altered clinical features associated with Omicron infection, the CDC said.

"The five reinfections, including one after full vaccination, might be explained by waning immunity, the potential for partial immune evasion by Omicron, or both," Jansen said, while adding that more data will be needed to fully understand the epidemiology of the Omicron variant.