(CNN)The flu season may be winding down, but parents of young children have reason to remain watchful.
As flu activity continued to decrease across the nation, the A-strain H3N2 influenza virus, which had dominated previously, was reported less frequently than B viruses, the US Centers for Disease Control and Prevention's weekly surveillance report indicated Friday.
During the week ending March 17, nearly 58% of all laboratory-confirmed cases of flu were caused by B-strain viruses, according to the CDC report. Circulating strains this season, which began in October, were a mix of A viruses (H3N2 and H1N1) and B viruses.
Flu is a contagious respiratory illness with mild to severe symptoms that can sometimes lead to death. Generally, the H3N2 strain leads to more severe illness and more hospitalizations than B strains, according to the CDC.
Yet parents might want to continue their vigilance with regard to younger children, CDC spokeswoman Kristen Nordlund suggested.
"We know that illness associated with influenza B can be just as severe as illness associated with influenza A," Nordlund said. "We also know that influenza B tends to be more severe for younger children."
The CDC recommends flu shots for the unvaccinated while strains continue to circulate. It is possible for people who've been sick with one strain of the flu to get a different strain in the same season.
Though flu activity is declining, this second wave of influenza B cases is not unexpected, Nordlund said. "We often see a wave of influenza B during seasons when influenza A H3N2 was the predominant virus earlier in the season. Unfortunately, we don't know what the influenza B wave will look like."
We may see a smaller second wave of B than in past seasons, but "we won't know until we have more data," Nordlund said.
During the 11th week of the year, the overall percentage of respiratory specimens testing positive in laboratories decreased compared with last week. Still, for the week, 4,759 specimens tested positive for flu, bringing the season's total to 246,766 positive specimens.
The CDC reported five additional flu-related pediatric deaths during week 11. In the 2017-18 season, 133 children have died.
Among adults, 7.8% of all deaths reported for the week were pneumonia- and flu-related, the CDC reported, noting that these data are always two weeks delayed. This rate is only slightly higher than the 7.4% threshold that had been anticipated for the week.
Nationwide, 2.7% of patients visiting their doctors complained of flu-like illness during week 11, just 0.5% higher than expected.
The overall hospitalization rate for the season is now 93.5 per 100,000 population. Among 26,694 total hospitalizations reported, nearly 78% were associated with influenza A virus.