2009 H1N1 Influenza


Vaccination is the most effective means of preventing influenza-associated morbidity and mortality. Because previous seasonal vaccinations do not appear to confer protection against 2009 H1N1, new vaccines have been licensed and are available. The manufacturing and licensure process for this vaccine was based on the same standards as the seasonal influenza vaccines. The vaccine is based on the A/California/07/2009 (H1N1) strain and is available in both live-attenuated and inactivated formulations. Given the prior broad H1N1 infection experience in the population, a single dose is adequate for those older than 9 years. With a single administration of the 2009 H1N1 vaccine, a robust immune response was seen in 80% to 96% of adults aged 18 to 64 years and in 56% to 80% of adults aged 65 years or older. Children younger than 10 years will require 2 administrations of the vaccine separated by at least 21 days. Although clinicians are advised to start providing the 2009-2010 seasonal vaccine as soon as it becomes available, some patients will likely present for vaccination for both seasonal and 2009 H1N1 influenza. Although the 2009 H1N1 vaccine in either form can be given simultaneously with the inactivated 2009-2010 seasonal vaccine, live-attenuated versions of both vaccines should not be used simultaneously. The live-attenuated vaccine is only licensed for persons aged 2 through 49 years who are not pregnant, are not immunocompromised, and have no chronic medical conditions. Children younger than 5 years who have asthma, those in close contact with immunosuppressed persons, and children receiving long-term aspirin therapy should not receive the live vaccine. The inactivated vaccine should not be given to patients with severe allergic reaction to eggs or any component of the vaccine.

In early studies, the immunogenicity and safety of the 2009 H1N1 vaccine have been similar to the seasonal influenza vaccine. Concerns regarding the risk of Guillain-Barre’ syndrome (GBS) after influenza vaccination have been raised. These concerns stem from the suspension of the 1976 H1N1 National Influenza Immunization Program because of reports of vaccine-related GBS. Subsequent analysis estimated an attributable risk of developing vaccine-related GBS from the 1976 H1N1 vaccine at just less than 1 per 100,000 persons in the adult population. Studies have been unable to show a consistent association between GBS and influenza vaccination, and studies suggest a higher risk of GBS from influenza itself rather than from the vaccine. Patients should be advised that adverse effects from the 2009 H1N1 vaccine are expected to be similar to those of the seasonal vaccine and notably involve the possibility of self-limited tenderness at the injection site of the inactivated vaccine and nasal congestion, rhinorrhea, or cough with the live-attenuated vaccine.

On July 29, 2009, the CDC’s Advisory Committee on Immunization Practices convened to review current epidemiological and clinical data to establish priority groups to receive 2009 H1N1 vaccine. Five such groups were established: pregnant women, caregivers for and cohabitants of infants younger than 6 months, health care and emergency services personnel, persons aged 6 months through 24 years, and persons aged 25 through 64 years with medical conditions that increase risk of influenza-related complications. Members of these 5 groups constitute approximately half of the US population and are at higher risk of influenza-related complications, have an occupational risk of acquiring and subsequently transmitting influenza, or are contacts of infants younger than 6 months who are too young to be vaccinated.

If vaccine production does not meet the demand for these 5 priority groups, the Advisory Committee on Immunization Practices has designated 5 population subsets for highest priority in receiving vaccine: pregnant women, caregivers for and cohabitants of infants younger than 6 months, health care and emergency services personnel in direct contact with patients or infectious material, children aged 6 months to 4 years, and children or adolescents aged 5 to 18 years at higher risk of influenza-related complications. In the event of any local vaccine shortages, members of these highest priority subsets, numbering approximately 42 million in the United States, should be vaccinated first. Once demand among all high-priority groups has been met, vaccination should be extended to other persons aged 25 through 64 years, followed by those older than 65 years.

Unfortunately, supplies of the 2009 H1N1 vaccine have been limited. Several reasons for the shortage can be identified (http://cdc.gov/h1n1flu/vaccination/qa_vac_supply.htm). Not only has the virus necessary for vaccine production grown more slowly than expected, larger than anticipated quantities of the virus were necessary to produce an acceptably potent vaccine. These issues have been addressed and, as of December 1, 2009, nearly 60 million doses of vaccine had been shipped from the manufacturers for distribution (http://cdc.gov/h1n1flu/vaccination/vaccinesupply.htm).

According to an Internet-based survey of 2067 US adults conducted by the RAND corporation between May 26, 2009, and June 8, 2009, approximately half (49.6%) of the respondents indicated that they intended to receive the 2009 H1N1 vaccine when it became available. Intention to receive the vaccine was strongly correlated with receipt of the seasonal influenza vaccine. Similarly, anonymous questionnaires completed by 2255 health care professionals in Hong Kong indicated that only 47.9% were willing to accept 2009 H1N1 influenza vaccine. In this study, willingness to receive pandemic influenza vaccination was not affected by changes in the WHO pandemic alert level.

Patients should be educated regarding other preventive measures, including using tissues to cover their mouth and nose when coughing and sneezing, developing good handwashing technique, using alcohol-based hand cleaners, avoiding contact with ill persons if possible, and staying home when ill unless medical attention must be sought.

The CDC has provided recommendations for infection control of 2009 H1N1 in health care settings. In order of importance, they are as follows: elimination of potential exposures, use of engineering controls (ie, partitions in triage areas), administrative controls (ie, promoting and providing vaccinations and enforcing policies for ill staff members), and use of personal protective equipment. Standard isolation precautions, including the use of nonsterile gloves for any patient contact and the use of gown and eye protection for activities involving respiratory secretions or other infectious material, should also be observed. Aerosol-generating procedures should ideally be performed in an airborne infection isolation room with as few people present as necessary. Isolation precautions should be continued for 7 days after illness onset in confirmed or suspected cases in whom 2009 H1N1 infection has not been excluded. If the patient is transported from the room, he or she should wear a mask if tolerable and should be encouraged to perform hand and cough hygiene practices frequently. Health care personnel should observe standard precautions and wear respiratory protection equivalent to a fitted N95 respirator plus eye protection during aerosol-generating procedures.

After review of the evidence regarding the efficacy of various types of available respiratory protective equipment, the Institute of Medicine has recommended that health care workers use fit-tested N95 respirators or the equivalent when in close contact with persons who have ILIs or are infected with the 2009 H1N1 virus. Supporting this recommendation are data that fit-tested N95 respirators filter 95% to 99% of infectious particles, whereas variable filtration results have been reported for surgical masks. Despite this, perhaps for economic considerations, some institutions have implemented policies that call for the use of masks for routine patient care activities and reserve N95 masks for aerosol-generating activities only.

At the end of the patient encounter, immediately after removing gloves, health care workers should wash their hands with soap and water or alcohol-based sanitizer. Health care personnel with febrile respiratory illness should notify their supervisor or infection control officer and be instructed not to work or to avoid patient care duties for 7 days from illness onset and until they have been asymptomatic for 24 hours, whichever is longer. Finally, all health care personnel should receive both seasonal and pandemic influenza vaccines.

Readers should be advised that these infection control recommendations are subject to rapid change and should consult their local health departments and infection control officers for further guidance.

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