These supplemental recommendations provide guidelines for a monovalent influenza A(H1N1) vaccine for protection against a newly emerged variant of influenza that has recently caused outbreaks among children and young adults in Asia. Guidance is provided for the use of this monovalent vaccine, which contains 15 ug of A/Taiwan/1/86(H1N1) antigen, as a supplement to the standard trivalent influenza vaccine. Recommendations for the use of the standard trivalent influenza vaccine for the 1986-1987 season and the use of antivirals for the prevention and treatment of influenza (MMWR 1986;35:317-26, 331) remain in effect and should be referred to in conjunction with this supplemental recommendation. The trivalent vaccine is intended to protect against currently circulating strains of influenza A(H3N2) and influenza B viruses and may provide partial protection against the new influenza A(H1N1) variant. *

INTRODUCTION

Influenza A(H1N1) viruses circulated throughout the world from at least the mid-1930s until 1957, and many epidemics during this period were associated with severe illness and excess mortality (1). Influenza A(H1N1) viruses similar to a strain seen in 1950 reappeared in epidemic form in 1977, but outbreaks were detected only among children and young adults. In 1978-1979, when a U.S. epidemic was caused exclusively by type A(H1N1) virus, widespread outbreaks occurred among children and young adults, but no excess mortality was observed at the national level (1).

Influenza A(H1N1) viruses, like other human influenza viruses, have continued to undergo antigenic variation and have caused outbreaks in the United States during several winters, most recently that of 1983-1984. Since 1977, the incidence of illness associated with influenza A(H1N1) infection has been very low among older adults; such illnesses have generally been mild (2); and virtually no outbreaks have been detected among older age groups, even though the post-1977 antigenic variants have differed from those that circulated before 1957 (3). A temporal relationship between the occurrence of influenza A(H1N1) infections in the community and increased hospitalizations of older persons for acute respiratory disease (ARD) has been reported in one investigation (4); however, the severity of ARD (e.g., incidence of pneumonia) and the excess number of hospitalizations for ARD associated with influenza are not known. Furthermore, from 1982 to 1986, the laboratories collaborating in CDC's influenza virus surveillance program reported 1,049 influenza type A(H1N1) virus isolates, of which only six (0.6%) were obtained from persons aged 65 years or older. During the same period, 566 (22%) of 2,635 type A(H3N2) and 169 (9%) of 1,905 type B viruses were isolated from persons in this age group. This indicates that, although older Americans have had repeated exposure to all three currently circulating influenza strains, they do not have the same level of natural protection against illness caused by new variants of type A(H3N2) or type B viruses as they do against new variants of type A(H1N1) virus. Thus, it appears that, in influenza A(H1N1) epidemics since 1977, children and young adults have been particularly at risk of infection and illness and that the frequency of illness has decreased markedly among persons born before the mid-1950s. Nevertheless, some persons born before this time remain susceptible to infection and may have respiratory illnesses requiring medical attention.

Following the 1983-1984 influenza season, A(H1N1) strains were isolated infrequently in most parts of the world. The majority of A(H1N1) isolates in 1984 and 1985 continued to resemble the A/Chile/1/83 strain (which was first included in the trivalent influenza vaccine for 1984-1985), and A/Chile/1/83 was, therefore, chosen to remain the A(H1N1) component for the trivalent vaccine previously recommended for 1986-1987 (5). However, A(H1N1) viruses from influenza outbreaks in several Asian countries during March-May 1986 have recently been found to be poorly inhibited by antibody induced by the A/Chile/1/83 strain. In contrast, these viruses were all well inhibited by antisera to representatives of the new isolates. In addition, tests of antibody response induced by A/Chile/1/83 vaccine among children or adults showed four- to sixfold lower postvaccination geometric mean titers against representatives of the new variants than against A/Chile/1/83 (6,7).

It is not possible to predict how widely these new A(H1N1) variants will circulate in the United States during 1986-1987, nor the actual level of protection that A/Chile/1/83 vaccine will induce against them. However, it seems prudent to maximize protection of individuals at high risk of serious complications following influenza A(H1N1) infection in the event that these newer A(H1N1) viruses do cause major outbreaks in the United States. Vaccine manufacturers have, therefore, been requested to initiate production of a supplemental monovalent A(H1N1) influenza vaccine for use before the 1986-1987 season.

Influenza A(H3N2) and type B viruses closely related to the strains in the 1986-1987 vaccine have continued to circulate throughout the world and may also appear in the United States during the 1986-1987 influenza season. The supplemental influenza A(H1N1) vaccine, unlike the 1986-1987 trivalent vaccine, will not contain representative antigens for virus types A(H3N2) and B. It is, therefore, imperative that the trivalent vaccine continue to be used as previously recommended (5). Programs for administration of the 1986-1987 trivalent vaccine to high-priority target groups should not be delayed, regardless of the time of availability of the supplemental A(H1N1) vaccine.

RECOMMENDATION

Individuals under 35 years of age for whom influenza vaccination has been specifically recommended (5) should receive both the standard trivalent vaccine and the monovalent A/Taiwan/1/86(H1N1) vaccine.

Any high-risk person aged 35 years and older, or any other person who wishes to be immunized, may also receive the supplemental vaccine.

PERSONS WHO SHOULD NOT BE VACCINATED

Inactivated influenza vaccine of any kind should not be given to persons who have an anaphylactic sensitivity to eggs. Persons with acute febrile illnesses should not be vaccinated until their temporary symptoms have abated. For recommendations regarding the use of influenza vaccine during pregnancy, refer to the previously published recommendations for the control of influenza (5).

TIMING OF INFLUENZA VACCINATION ACTIVITIES

Recommendations for the timing of influenza vaccination activities with the trivalent vaccine for use in 1986-1987 have been published (5). Those recommendations remain in effect. Additional recommendations below (Table 5) apply to persons receiving the supplemental A(H1N1) vaccine in conjunction with the 1986-1987 trivalent vaccine.

Children aged 12 years or younger who have never received any influenza vaccine containing type A(H1N1) antigen (i.e., any influenza vaccine since 1978-1979) are considered unprimed and require two doses of the standard trivalent vaccine with an interval of at least 4 weeks between doses. The timing and number of monovalent A(H1N1) vaccine doses required will vary depending on whether the recipient has been primed by prior vaccination or infection and on the timing of doses administered for the current season (Table 5).

If the supplemental monovalent vaccine is not available at the time vaccination programs would normally be undertaken, vaccination with the standard trivalent vaccine should not be delayed.

It is anticipated that the supplemental monovalent vaccine will not be available until November-December 1986. If influenza A outbreaks begin to occur before vaccination, temporary chemoprophylaxis with the antiviral agent, amantadine, may be indicated. Recommendations for amantadine use for prophylaxis and treatment of influenza A infections have been published (5).

Information about the availability of the supplemental vaccine and the occurrence of influenza will be made available to state health officials by electronic communication and will be published in the MMWR.

RECOMMENDED DOSAGE OF SUPPLEMENTAL MONOVALENT INFLUENZA VACCINE

The 1986-1987 supplemental monovalent vaccine contains 15 ug of A/Taiwan/1/86 antigen in each 0.5-ml dose. As with the standard trivalent vaccine, the recommended dosage of the monovalent vaccine should be reduced to 0.25 ml for children 6-35 months of age. Only split-virus vaccine, suitable for use in children or adults, will be manufactured. When administered simultaneously with the 1986-1987 trivalent vaccine, the vaccines should be given in separate sites (e.g., right and left deltoid or thigh). For more specific information, see the recommendations for 1986-1987 (5).

SIDE EFFECTS AND ADVERSE REACTIONS

Children aged 6-35 months will receive a total of 30.0 ug of antigen when given both vaccines simultaneously, compared with 22.5 ug when given trivalent influenza vaccine alone; children 3 years of age or older and adults will receive a total of 60.0 ug of antigen when given both vaccines simultaneously, 45.0 ug when given only the trivalent vaccine. Studies of the effect of different doses of influenza vaccine antigen administered to children and adults suggest that the amounts of antigen delivered by simultaneous administration of the trivalent and monovalent vaccines will result in no significant differences in the occurrence or severity of systemic adverse reactions compared with administration of trivalent vaccine alone (8-10).

More information on side effects and adverse reactions associated with inactivated influenza vaccine has been published (5).

References

1. Noble GR. Epidemiological and clinical aspects of influenza. In: Beare AS, ed. Basic and applied influenza research. Boca Raton: CRC Press, Inc., 1982:11-50.

2. Monto AS, Koopman JS, Longini IM, Jr. Tecumseh study of illness. XIII. Influenza infection and disease, 1976-1981. Am J Epidemiol 1985;121: 811-22.

3. Raymond FL, Caton AJ, Cox NJ, Kendal AP, Brownlee GG. The antigenicity and evolution of influenza H1 Haemagglutinin, from 1950-1957 and 1977-1983: two pathways from one gene. Virology 1986;148:275-87.

4. Perrotta DM, Decker M, Glezen WP. Acute respiratory disease hospital- izations as a measure of impact of epidemic influenza. Am J Epidemiol 1985;122:468-76.

5. ACIP. Prevention and control of influenza. MMWR 1986;36:317-26, 331.

6. CDC. Antigenic variation of recent influenza A(H1N1) viruses. MMWR 1986;35:510-12.

7. World Health Organization. Composition of influenza virus vaccines for the 1986-87 season: an update. Wkly Epidem Rec 1986;31:237-8.

8. La Montagne JR, Noble GR, Quinnan GV, et al. Summary of clinical trials of inactivated influenza vaccine -- 1978. Rev Infect Dis 1983;5:723-36.

9. Gross PA, Quinnan GV, Gaerlan PF, et al. Potential for single high-dose influenza immunization in unprimed children. Pediatrics 1982;70:982-6.

10. Arden NA, Patriarca PA, Lui KJ, Harmon MW, Brandon F, Kendal AP. Safety and immunogenicity of a 45-ug supplemental dose of inactivated split- virus influenza B vaccine in the elderly {Letter}. J Infect Dis 1986; 153:805-6.

Product information about influenza vaccines can be obtained from the following manufacturers: Connaught (vaccine distributed by Squibb) -- (609) 921-4000; Parke-Davis -- (800) 223-0432; Wyeth -- (800) 321-2304.

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