Influenza Hospitalizations During Pregnancy in Suzhou, China, 2018–2023

About 200 million women become pregnant worldwide each year, and among women of reproductive age with influenza, pregnant women are the most likely to become seriously ill. Many countries, including China, recommend flu vaccination during pregnancy, yet coverage is often low — in part because little is known about how often pregnant women are actually hospitalized with flu. A study in Suzhou, a city of about 13 million in southern China's Jiangsu Province, set out to measure it, working with CDC.

How the study worked

From October 2018 to September 2023, active surveillance covered every medical institution in Suzhou. It identified hospital admissions for acute respiratory or febrile illness among women of reproductive age, using a wide range of diagnosis codes and a low fever threshold (99.1°F, or 37.3°C) to catch as many flu-like illnesses as possible. Pregnant women and those who had given birth within the previous two weeks had nasal swabs tested by RT-PCR for influenza. Rates were calculated per 1,000 live births.

Findings

• Hospitalizations: 3,329 pregnant or early postpartum women were hospitalized with acute respiratory or febrile illness, and 3,133 were tested. Of those, 495 (15.8%) had influenza.
• Viruses: 65.7% had influenza A — split almost evenly between H3N2 and H1N1pdm09 — and 33.7% influenza B, nearly all of the Victoria lineage.
• Timing: 79.2% of influenza cases came in the third trimester, 10.7% in the first, 8.1% in the second and 2.0% after delivery; 96.8% occurred when flu was at epidemic levels.
• Rates: hospitalization for any respiratory or febrile illness averaged 11.1 per 1,000 live births a year. Influenza hospitalization averaged 2.1 per 1,000 live births — excluding 2020–2021, when COVID-19 measures such as distancing pushed flu to almost nothing (0.05 per 1,000). The highest year was 2018–2019, at 3.4 per 1,000.
• Counting unrecorded cases: applying findings from a 2022 Suzhou cohort study, total flu illness among pregnant and postpartum women, including cases that never reached a doctor, was estimated at 65.9 per 1,000 live births a year — comparable to studies in El Salvador, Panama and Kenya.

Where they were treated

Most women with influenza (85.5%) were admitted to obstetric wards, and only 7.5% to respiratory medicine wards. That matters because China's sentinel flu surveillance mainly monitors respiratory wards and does not record pregnancy; relying on it would have missed about 85% of these hospitalizations. Nine women with influenza (1.8%) needed intensive care; none needed mechanical ventilation, and none died in the hospital.

Vaccination and treatment

• Vaccination: just one of the 3,329 women had received a flu vaccine, and she tested negative for influenza. Nationally, coverage among China's roughly 18 million pregnant women a year is 0.04%. Based on vaccine effectiveness data, about 40% of these hospitalizations might have been prevented.
• Antivirals: only 31.3% of women with influenza received antiviral treatment — 29.6% of those on obstetric wards compared with 54.1% on respiratory wards. Cost, limited availability and worries about side effects or harm to the fetus may play a part, although observational studies of oseltamivir and zanamivir in pregnancy have not shown risk to the fetus.

What could help

The authors recommend including maternity and postnatal wards in flu surveillance; educating obstetricians about the risks of flu in pregnancy and encouraging them to strongly recommend vaccination; and training providers, especially on obstetric wards, to treat pregnant and postpartum women with influenza using antivirals. The estimates could also inform cost-benefit analyses of vaccines, antivirals and preventive measures such as respiratory hygiene and avoiding crowds.

Sources

• Jinghui Sun, Yuanyuan Zhang, Suizan Zhou, Ying Song and colleagues, "Laboratory-Confirmed Influenza Hospitalizations During Pregnancy or the Early Postpartum Period — Suzhou City, Jiangsu Province, China, 2018–2023," Morbidity and Mortality Weekly Report, CDC: https://www.cdc.gov/mmwr/volumes/73/wr/mm7343a1.htm
• The report's figure is not reproduced.