The city reported 309 cases across 65 wards and communes between August 14 and 21, along with 15 new clusters, according to the Hanoi Centre for Disease Control.
Health officials said seasonal conditions, unpredictable weather and the return of large numbers of students to the capital could increase the risk of outbreaks.
The Hanoi Department of Health has instructed hospitals to strengthen screening, admissions, emergency care and treatment, while ensuring adequate supplies of medicines, intravenous fluids, blood and medical equipment.
Thuong Tin General Hospital, which has treated dozens of dengue patients on some days, has 300 beds and can expand to about 500 if necessary. It has also prepared additional treatment areas in case patient numbers surge.
Soc Son General Hospital has stepped up patient screening and staff training to detect severe cases early and reduce the risk of complications.
Doctors warned that the most dangerous stage of dengue can occur when fever begins to subside, typically between the third and seventh day after symptoms appear.
Patients should seek immediate medical attention if they experience warning signs including severe abdominal pain, persistent vomiting, bleeding, lethargy, breathing difficulties, reduced urination or dizziness. People with fever were advised against self-medicating or administering intravenous fluids at home.
Residents were also urged to eliminate mosquito breeding sites, remove standing water, cover water containers and take precautions against mosquito bites.
