11 April 2006

WHO Calculates Large Numbers of Masks Needed for Patients and Health Providers During a Flu Pandemic

The World Health Organization (WHO) posted on their website April 7^th a pragmatic 53-page document that provides guidelines for humanitarian agencies titled, “Pandemic influenza preparedness and mitigation in refugee and displaced populations”.

New, explicit guidance is provided on collection of diagnostic patient specimens (page 14). For example, the emerging evidence from WHO that throat swabs are proving more useful that with seasonal flu in making the laboratory diagnosis of H5N1 is reflected in the WHO advice that: “Nasopharyngeal and throat sampling of tonsils and posterior pharynx with a Dacron swab is recommended.

The annex section of this document gives clinically useful information on infection control issues, disinfectants, the dosing of oseltamivir (Tamiflu), and how to calculate the number of surgical masks needed during a pandemic. In annex 6 (page 45) WHO estimates that in a sample population of 10,000 persons it should be anticipated that 3,000-5,000 will seek health care over the course of the pandemic and that 500 inpatient admissions should be expected.

Surgical Masks Calculations for patients with Influenza-Like Illness (ILI):

This calculation is based on two estimates: (1) Persons being initially evaluated and (2) inpatients. WHO recommends one surgical mask for each patient with influenza-like illness who seeks evaluation. Therefore, 3,000-5,000 surgical masks will be needed for this first part of the calculation. WHO estimates that for the 400-500 inpatients one mask per day for ~ 7 days will be needed. Thus, a total of 2,800-3,500 surgical masks are needed for inpatients to decrease the spread of influenza virus particularly during coughing or sneezing.

Thus, the total number of surgical masks for symptomatic patients in this 10,000 person sample population is between (3,000-5,000) + (2,800-3,500) = 5,800-8,500 masks.

Surgical Mask Calculations for Health Care Workers (HCWs) and other Staff:

This calculation is also based on two estimates: (1) HCWs, and (2) other potentially exposed staff. The HCW calculation is based on a WHO estimate of 4 mask changes per day, times the number of health staff, times the duration in days of the pandemic (90-180 days), PLUS a 50% buffer stock of surgical masks. Thus, 4 x 90= 360 + 50% buffer stock (i.e., 180 more masks) = a minimum estimate of 540 masks PER HCW assuming a 90 day pandemic duration. If the pandemic duration of 180 days is used instead of 90 days, then twice as many surgical masks will be needed, i.e., 1080 masks PER HCW.

For other potentially exposed staff such as food and water handlers, cleaners, and security guards, the WHO estimates that only two mask changes per day will be needed rather than the four mask changes per day for HCWs. Thus, for a 90 day pandemic 270 surgical masks PER PERSON would be needed. For a 180 day pandemic, 540 masks would be needed PER PERSON.

For HCWs WHO recommends that surgical masks “should be thrown away when leaving isolation wards, OR every 4 hours or when wet or visibly soiled” (page 24).

For other essential staff without direct patient care responsibilities surgical (or procedural) masks should be changed “every 4 hours, or when wet or visibly soiled” (page 24).

This is the first guidance and rationale on how to estimate the number of surgical masks both for patients and for persons involved directly and indirectly in their care during an influenza pandemic. The resulting total number of masks needed in nations with tens or hundreds of millions of people is very large.

Estimates of the number of (fit-tested) N-95 particulate respirators, considerably more expensive than surgical masks, is not provided, although WHO recommends such particulate respirators for any type of aerosol-generating procedure.

These calculations of surgical masks numbers for the next flu pandemic should be evaluated within national, regional, and local contexts, including the announcement March 13^th by HHS Secretary Leavitt that the US plans this year to purchase 50 million surgical masks (and 50 million N-95 particulate respirators).

Daniel R. Lucey, MD, MPH

Director, Center for Biologic Counterterrorism and Emerging Diseases

Washington Hospital Center, EROne Institutes

Co-Director, Master of Science Program in Biohazardous Threat Agents and Emerging Infectious Diseases, Georgetown University School of Medicine

Washington, DC

Website: www.BePast.org

Email:Daniel.R.Lucey@Medstar.net