Pregnant women may be at higher risk from the flu this season, but it appears to have more to do with falling vaccination rates than with the virus itself.
Dr. Michelle Barron, senior director of infection prevention and control at UCHealth, said about half of the system’s female patients between the ages of 18 and 44 have been pregnant so far this year. During the 2019-2020 flu season — the last normal one before the pandemic — only about 17% were, which is more typical, she said.
“Seventeen percent to 50% is a huge jump, based on historically what we’ve seen,” she said.
Preliminary data from the Centers for Disease Control and Prevention also shows a disproportionate share of hospitalized patients of childbearing age were pregnant. The CDC defines patients with childbearing potential as girls and women ages 15 to 49, and has found about 40% of those hospitalized in that group were pregnant at the time they got sick.
In previous seasons, the CDC only collected data on pregnancy up to age 44, so comparisons over time aren’t exact. Still, it appears the odds a severely ill patient will be pregnant have increased compared to the 2018-2019 flu season, when about 26% of those hospitalized in that group were. The data comes from 14 states, including Colorado.
The CDC estimated about 47% of people who were pregnant as of December had received the flu shot, down from 62% in December 2019. The estimates are based on electronic health records from nine systems, however, so they may not represent the situation everywhere.
This season’s dominant flu strain, H3N2, is one that the world has seen before and which isn’t specifically worse for pregnant people, Barron said. That suggests low rates of flu vaccination may be driving the difference, since misinformation about the COVID-19 vaccine spilled over into general hesitancy among pregnant women, she said.
“Flu is here, and is behaving like it did before,” she said.
During pregnancy, the body suppresses its T cells, the part of the immune system that identifies invaders and tries to destroy them, Barron said. That prevents the immune system from attacking the fetus, but leaves both mother and baby more vulnerable to infections, she said.
People who are pregnant were more vulnerable to severe COVID-19 than nonpregnant people with the same age and chronic conditions (or lack thereof), and certain types of food poisoning are particularly risky during pregnancy.
“The number of deaths in pregnant people from COVID was mortifying, because these are young, healthy people,” she said.
The B cells, which produce antibodies, continue to work well throughout pregnancy, though. That means that a pregnant person will typically develop a normal protective response after being vaccinated, Barron said. While there are a handful of vaccines that pregnant women can’t receive because they contain a live virus, the flu shot isn’t one of them, and it’s recommended for anyone who is pregnant, she said.
Not all Colorado hospitals saw an increase in pregnant patients with flu.
Denver Health reported the number of pregnant women with flu in its hospital was in line with previous years. HealthOne, which owns seven general hospitals in the Denver area, reported a small increase in pregnant flu patients earlier this season at Rose Medical Center, but said the other locations have seen typical volumes.
Dr. Susan Lipinski, an obstetrician-gynecologist at Presbyterian/St. Luke’s Medical Center, said she encourages anyone who is pregnant or planning to be in the near future to get the flu shot as soon as possible, even though it’s now later in the season.
The shot reduces the odds of complications for both the mother and the fetus and offers the baby some protection after birth, she said. Babies can receive the flu shot once they’re 6 months old, but are at elevated risk from the virus until that time.
“The flu vaccine is safe in any trimester of pregnancy,” she said. “I have taken care of patients in March or April (the last months of flu season) with severe influenza.”
In addition to altering the immune system, pregnancy puts extra stress on the lungs, which makes respiratory viruses particularly risky, Lipinsky said. The lungs are being asked to take in more oxygen to support the fetus, while at the same time they have less room to expand because of pressure from the growing uterus, she said.
In the worst cases, the baby has to be delivered early to give the mother’s body a better chance of fighting the virus, Lipinsky said. Even in less-severe cases, a fever increases the odds of birth defects, particularly if it comes during the first trimester, when important organs are forming.
In addition to getting the flu shot, people can protect themselves by avoiding others who are sick, wearing masks in crowds, washing their hands frequently and seeing a doctor about antiviral treatment if they get infected, Barron said. While the current flu wave is going down, it wouldn’t be unusual for a different strain to start circulating later in the winter, she said.
“Flu season is not over,” she said.
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