Influenza (Flu) Activity

Information and resources on influenza (flu) activity in New Hampshire

Influenza (Flu) is not a reportable disease in New Hampshire, yet surveillance (data) systems are in place to determine the reach of illness and current virus types being spread.

Weekly Influenza Activity Report

Timing: During the traditional influenza season (beginning of October through mid-May).

What data is included?

Data from heath care providers and laboratories
Data from other state and national surveillance systems.
NH influenza activity for the week.

New Hampshire participates in the US Outpatient Influenza-like Illness Surveillance Network (ILINet), a collaborative effort between the CDC, the NH Department of Health and Human Services, and health care providers throughout the State, and surveillance data reported by ILINet sentinel providers is included in the weekly reports.

Weekly Report

Upon conclusion of the traditional influenza season, surveillance data is summarized in the form of an annual season summary report.

Season Summary Report

This report summarizes:

The severity of the most recent influenza season, including
When influenza activity peaked based on different surveillance measures, such as:
- Acute respiratory illness,
- Number respiratory specimens that test positive for influenza (by influenza type), and
- The total number of influenza-related deaths.

Surveillance data are summarized to determine which age groups are most affected.

Data are compared to the previous influenza season and to regional and national surveillance data to help put the data into perspective.

In addition to surveillance data presented in the Season Summary Report, information on annual vaccination rates among healthcare personnel in hospitals, ambulatory surgical centers, and assisted living facilities can be viewed by visiting the Healthcare-Associated Infections Program.

Any suspect outbreak, cluster of illness, unusual occurrence of communicable disease, or other incident that may pose a threat to the public’s health must be reported within 24 hours of recognition. Any infections with novel or avian influenza, as well as pediatric influenza-related deaths, would be considered “implicitly” reportable.

Flu Season Summary Reports

NH Acute Respiratory Illness (ARI) Surveillance by County

Below are maps for each week during the usual flu season, which begins in early October through mid-May.

The maps show the how intense the Acute Respiratory Illness (ARI) is for each of the ten New Hampshire counties.

2024-2025 Season

2023-2024 Season

2022-2023 Season

The Influenza (Flu) Surveillance (Data) Program watches flu activity in NH year round. However, the weekly flu activity reports are only completed and posted during the course of the usual flu season, early October through mid-May of the following year.

What data is included?

Key components for influenza surveillance are as follows:

Influenza (Flu) Virologic Surveillance

Influenza (Flu) test results are completed and reported out from two main sources in NH, the NH Public Health Laboratories (NH PHL) and Hospital Clinical Laboratories.

NH Public Health Laboratories (NH PHL)

Samples collected from patients in outpatient settings (hospitals and doctor offices) and institutional settings (long-term care facilities) are submitted to the NH PHL for testing.

The NH PHL tests samples using a method called reverse transcription-polymerase chain reaction (RT-PCR) assay, which determines both the flu type (e.g., flu A or B) and flu subtype [e.g., influenza A(H3) or A(H1N1)].

Hospital Clinical Laboratories

Hospital Clinical Laboratories submit weekly reports for flu results.

The weekly reports are a summary of flu test results from participating hospital clinical laboratories across the state and include:

The total number of samples tested for flu each week, and
The number of positive tests for either flu A or flu B.

These data are summarized weekly, year-round, in the influenza surveillance reports published during the usual flu season, early October through mid-May.

Outpatient Illness Surveillance

Outpatient Illness Surveillance is performed on a weekly basis throughout the year by looking at two different sources of data, including:

U.S. Outpatient Influenza-Like Illness Surveillance Network (ILINet)
ILINet is a collaborative effort between the CDC, the NH Division of Public Health Services (NH DPHS), and healthcare providers throughout the State. For this effort, ILI is defined as fever (temperature of 100°F [37.8°C] or greater) and a cough and/or a sore throat.
- More information about ILINet can be found here.
Automated Hospital Emergency Department Data (AHEDD) system
The AHEDD system is a joint effort between NH acute care hospitals and the NH DPHS. Currently, 26 hospitals submit real-time Emergency Department (ED) data throughout the day to NH DPHS. Using a method called syndromic surveillance, the main reason (for being in the ED) word(s) or text, such as acute respiratory illness, is searched for in the AHEDD system.

Flu Mortality Surveillance

The percentage of all deaths occurring each week due to pneumonia and influenza (P&I) are evaluated and reported weekly during the usual flu season (Weekly Influenza Activity Report).

Deaths due to P&I are identified through a review of the electronically filed death certificates for the causes of death listed on each certificate submitted to NH's Division of Vital Records Administration.

The number of flu-related deaths identified, from the death records, are reported weekly during the usual flu season.