Influenza-Related Maternal Morbidity Has More Than Doubled Over 15 Years

Tara Haelle

February 23, 2021

Despite slightly decreasing numbers of pregnant women hospitalized with influenza, the rate of morbidity among those who do have influenza has substantially increased from 2000 to 2015, likely due in part to an increase in comorbidities.

Maternal patients who have influenza while hospitalized for delivery are twice as likely to develop severe maternal morbidity than are those without influenza, according to findings from a new study presented at the Pregnancy Meeting, sponsored by the Society for Maternal-Fetal Medicine.

Pregnant women were also at substantially greater risk of sepsis or shock, needing mechanical ventilation, and acute respiratory distress syndrome. In fact, rates of overall severe maternal morbidity and of influenza-related complications have increased in maternal patients with influenza by more than 200% from 2000 to 2015.

"It was striking to see how the rate of delivery hospitalizations complicated by influenza has remained relatively stable with a small decline, but the rates of severe maternal morbidity were increasing and so markedly among those with influenza," Timothy Wen, MD, MPH, a maternal-fetal medicine clinical fellow at the University of California, San Francisco, said in an interview. "The findings suggest that influenza may either be a contributor to rising rates of severe maternal morbidity or synergistically amplifying existing comorbidities to worsen outcomes," he said during his presentation.

The increased risk of influenza complications in pregnant women became particularly apparent during the 2009-2010 H1N1 influenza pandemic. "Physiologic and immunologic changes predispose pregnant patients to higher risk for complications such as pneumonia, intensive care unit admission, and inpatient mortality," Wen told attendees. But data have been scarce since H1N1.

The researchers conducted a cross-sectional analysis of delivery hospitalizations from 2000 to 2015 using the Nationwide Inpatient Sample, which includes about 20% of all U.S. inpatient hospitalizations from all payers. They looked at all maternal patients aged 15-54 who had a diagnosis of influenza. In looking at potential associations between influenza and morbidity, they adjusted their calculations for maternal age, payer status, median income, and race/ethnicity as well as the hospital factors of location, teaching status, and region. They also adjusted for a dozen clinical factors.

Of 62.7 million hospitalizations, 0.67% involved severe maternal mortality, including the following influenza complications:

The 182,228 patients with influenza represented a rate of 29 cases per 10,000 deliveries, and 2.09% of them involved severe maternal morbidity, compared to severe maternal morbidity in just 0.66% of deliveries without influenza.

When looking specifically at rates of shock/sepsis, mechanical ventilation, and acute respiratory distress syndrome, the data revealed similar trends, with substantially higher proportions of patients with influenza experiencing these complications compared to maternal patients without influenza. For example, 0.3% of patients with influenza developed shock/sepsis whereas only 0.04% of patients without influenza did. Acute respiratory distress syndrome was similarly more common in patients with flu (0.45% vs. 0.04%), as was the need for mechanical ventilation (0.09% vs. 0.01%).

During the 15-year study period, the rate of maternal hospitalizations with influenza infections declined about 1.5%, from 30 to 24 per 10,000 deliveries. But trends with severe maternal morbidity in patients with influenza went in the other direction, increasing more than 200% over 15 years, from 100 to 342 cases of severe maternal morbidity per 10,000 patients with influenza. An increase also occurred in patients without influenza, but it was more modest, a nearly 50% increase, from 53 to 79 cases per 10,000 hospitalizations.

From year to year, severe maternal morbidity increased 5.3% annually among hospitalizations with influenza – more than twice the rate of a 2.4% annual increase among hospitalizations without influenza.

The researchers found that influenza is linked to twice the risk of severe maternal morbidity (adjusted risk ratio [aRR] = 2.08, P < .01). There were similarly higher risks with influenza of sepsis/shock (aRR = 3.23), mechanical ventilation (aRR = 6.04), and acute respiratory distress syndrome (aRR = 5.76; all P < .01).

Among the possible reasons for the increase in influenza morbidity – despite a decrease in influenza infections in this population – is the increase in the medical complexity of the patient population, Wen said.

"Patients who are getting pregnant today likely have more comorbid conditions (chronic hypertension, obesity, pregestational diabetes mellitus, etc.) than they did decades prior," Wen said. "Clinically, it means that we have a baseline patient population at a higher risk of susceptibility for influenza and its complications."

Maternal influenza immunization rates have meanwhile stagnated, Wen added. Influenza "is something that we know is preventable, or at least mitigated, by a vaccine," he said. "Our results serve as a reminder for clinicians to continue counseling on the importance of influenza vaccination among pregnant patients, and even in those who are planning to become pregnant."

He said these findings suggest the need for a low threshold for treating pregnant patients who have influenza symptoms with over-the-counter therapies or closely monitoring them.

Adetola Louis-Jacques, MD, of the University of South Florida, Tampa, found the increase in morbidity in those with flu particularly unexpected and concerning.

"What surprised me was the big difference in how severe maternal morbidity rates increased over time in the influenza group compared to the group without influenza," Louis-Jacques, who moderated the session, said in an interview. She agreed with Wen that the findings underscore the benefits of immunization.

"The study means we should reinforce to mothers how important the vaccine is. It's critical," Louis-Jacques said. "We should encourage mothers to get it and focus on educating women, trying to understand and allay [any concerns about the vaccine] and reinforce the importance of flu vaccination to decrease the likelihood of these mothers getting pretty sick during pregnancy."

Wen and Louis-Jacques had no disclosures.

This article originally appeared on MDedge.com, part of the Medscape Professional Network.

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