Age at Which Immunobiologics Are Administered
Factors that influence recommendations concerning the age at which a vaccine is administered include the age-specific risks of the disease and its complications, the ability of people of a given age to respond to the vaccine, and the potential interference with the immune response by passively transferred maternal antibody. Vaccines are recommended for the youngest age group at risk of developing the disease whose members are known to develop an adequate antibody response to vaccination.

The routine immunization recommendations and schedules for infants and children in the United States (Tables 1-2 and 1-3) do not provide specific guidelines for infants and young children who will travel internationally before the age when specific vaccines and toxoids are routinely recommended. The following section, "Immunization Schedule Modifications for International Travel for Inadequately Immunized Infants and Younger Children," provides revised recommendations and schedules for active and passive immunization of such infants and children.

Immunization Schedule Modifications for International Travel for Inadequately Immunized Infants and Younger Children

Routine Infant and Childhood Vaccine-Preventable Diseases (Diphtheria, Tetanus, Pertussis, Measles, Mumps, Rubella, Varicella, Polio, Haemophilus Influenzae Type b, and Hepatitis B)

Diphtheria and Tetanus Toxoid and Pertussis Vaccine
Diphtheria is an endemic disease in many developing countries and has been found in the independent countries of the former Soviet Union. Tetanus occurs worldwide.

Pertussis is common in developing countries and in other areas where pertussis immunization levels are low. Infants and children leaving the United States should be as well immunized as possible. Optimum protection against diphtheria, tetanus, and pertussis in the first year of life is achieved with three doses of diphtheria and tetanus toxoids and acellular pertussis vaccine (DTaP), the first administered when the infant is 6 to 8 weeks of age and the next two at 4- to 8-week intervals. A fourth dose of DTaP should be administered 6 to 12 months after the third dose when the infant is 15 to 18 months of age. A fifth (booster) dose is recommended when the child is 4 to 6 years of age. The fifth dose is not necessary if the fourth dose in the primary series was given after the child's fourth birthday. Two doses of DTaP received at intervals at least 4 weeks apart can provide some protection, particularly against diphtheria and tetanus; however, a single dose offers little protective benefit. Parents should be informed that infants and children who have not received at least three doses of DTaP might not be fully protected from pertussis. For infants and children younger than 7 years of age, if an accelerated schedule is required to complete the series prior to travel, the schedule may be started as soon as the infant is 6 weeks of age, with the second and third doses given 4 weeks after each preceding dose (see Note). The fourth dose should not be given before the infant is 12 months of age and should be separated from the third dose by at least 6 months. The fifth (booster) dose should not be given before the child is 4 years of age.

Measles Vaccine
Measles is an endemic disease in many developing countries and in other countries where measles immunization levels are low. Because the risk of contracting measles in many countries is greater than in the United States, infants and children should be as well protected as possible before leaving the United States. Infants and children who travel or live abroad should be vaccinated at an earlier age than is recommended for infants and children remaining in the United States. Before their departure from the United States, infants and children 12 months of age or older should have received two doses of measles, mumps, and rubella (MMR) vaccine separated by at least 28 days, with the first dose administered on or after the first birthday. Infants 6 through 11 months of age should receive a dose of monovalent measles vaccine before departure. If monovalent measles vaccine is not available, no specific contraindication exists to administering MMR to infants 6 through 11 months of age. However, because the risk for serious disease from either mumps or rubella infection among infants is relatively low and because infants younger than 12 months of age are less likely to develop serologic evidence of immunity when vaccinated with MMR antigens than are older infants and children, mumps and rubella vaccines generally are administered only to infants and children 12 months of age or older. Infants administered monovalent measles vaccine or MMR before their first birthday should be considered potentially susceptible to all three diseases and should be revaccinated with two doses of MMR, the first of which should be administered when the infant is 12 to 15 months of age (12 months if the infant remains in an area where disease risk is high) and the second at least 28 days later.

Parents who travel or reside abroad with infants younger than 12 months of age should have acceptable evidence of immunity to rubella and mumps, as well as measles, so they will not become infected if their infants contract these diseases. An infant younger than 6 months of age is usually

protected against measles, mumps, and rubella by maternally derived antibodies and ordinarily does not require additional protection unless his or her mother is diagnosed with measles.

Mumps and Rubella Vaccine(s)
Because the risk of serious disease from infection with either mumps or rubella in infants is low, mumps and rubella vaccine(s) generally should not be administered to infants younger than 12 months of age unless measles vaccine is indicated and single-antigen measles vaccine is not available. However, parents of an infant younger than 12 months of age should be immune to mumps and rubella so they will not expose the infant or become infected if the infant develops illness.

Varicella Vaccine
Varicella (chickenpox) is an endemic disease throughout the world. A single dose of varicella vaccine should be administered to all susceptible infants and children without contraindications at 12 months of age or older. Infants and children who have a reliable history of having had chickenpox do not need to be vaccinated. Infants younger than 12 months of age will generally be protected from varicella because of passive maternal antibody.

Polio Vaccine
Because OPV is no longer recommended for routine immunization in the United States, all infants and children should receive four doses of IPV at 2, 4, and 6 to 18 months and 4 to 6 years of age. If accelerated protection is needed, the minimum interval between doses should be 4 weeks, although the preferred interval between the second and third doses is 2 months. Infants and children who have initiated the poliovirus vaccination series with one or more doses of OPV should receive IPV to complete the series.

Haemophilus Influenzae Type b Conjugate Vaccine
Haemophilus influenzae type b (Hib) is an endemic disease worldwide. Risk of acquiring the disease might be higher in developing countries than in the United States. In the United States, four types of Hib conjugate vaccines are available, three of which may be used in infants beginning at 6 weeks of age. Two Hib conjugate vaccines for infants are also available as combined DTP-Hib vaccines. Routine Hib vaccination beginning at 2 months of age is recommended for all U.S. children. The first dose may be given when the infant is as young as 6 weeks of age. Hib vaccine should never be given to an infant younger than 6 weeks of age. A primary series consists of two or three doses (depending on the type of vaccine used) separated by 4 to 8 weeks. A booster dose is recommended when the infant is 12 to 15 months of age (see Travelers' Health Information on Haemophilus influenzae Type b Meningitis and Invasive Disease).

If vaccination is started when the infant or child is 7 months of age or older, fewer doses may be required. If different brands of vaccine are administered, a total of three doses of Hib conjugate vaccine completes the primary series. After completion of the primary infant vaccination series, any of the licensed Hib conjugate vaccines may be used for the booster dose when the infant is 12 to 15 months of age.

Infants and children should have optimal protection prior to travel. If previously unvaccinated, infants younger than 15 months of age should ideally receive at least two vaccine doses prior to travel. An interval as short as 4 weeks between these two doses is acceptable.

Unvaccinated infants and children 15 through 59 months of age should receive a single dose of Hib vaccine.

Hepatitis B Vaccine
Hepatitis B vaccine is recommended for all infants beginning either at birth or by 2 months of age. Infants and young children who have not previously been vaccinated and who are traveling to areas with intermediate and high hepatitis B virus (HBV) endemicity might be at risk if they are directly exposed to blood from the local population. Circumstances in which HBV transmission could occur include receipt of blood transfusions not screened for HBV surface antigen (HBsAg), exposure to unsterilized needles (or other medical or dental equipment) in local health facilities, or continuous close contact with local residents who have open skin lesions (impetigo, scabies, or scratched insect bites). Such exposures are most likely to occur if an infant or a child is living for long periods in smaller cities or rural areas and in close contact with the local population. Infants and children who will live in an area of intermediate or high HBV endemicity for at least 6 months and who are expected to have the preceding exposures should receive the three doses of HBV vaccine. The interval between doses one and two should be 1 to 2 months. Between doses two and three, the interval should be a minimum of 2 months; the third dose should not be given before the infant is 6 months of age. (See Table 1-2, for the suggested schedule and Table 3-7, for vaccine-specific doses.)

Other Vaccines and Immune Globulin

Typhoid Vaccine
Typhoid vaccination is not required for international travel. No data are available concerning the efficacy of typhoid vaccine in infants. Breast-feeding is likely to be protective against typhoid; careful preparation of formula and food from safe water and foodstuffs should protect non-breast-fed infants. Typhoid vaccine is recommended for children 2 years of age or older traveling to areas where there is a recognized risk of exposure to Salmonella typhi, particularly if they are traveling to highly endemic areas. (See Travelers' Health Information on Typhoid Fever for information on dosage and route of administration of the vaccines.)

 
Table 1-5.--Minimum Age for Initial Vaccination and Minimum Interval Between Vaccine Doses, by Type of Vaccine.

	Vaccine		Minimum Age for First Dose*		Minimum Interval from Dose 1 to 2*		Minimum Interval from Dose 2 to 3*		Minimum Interval from Dose 3 to 4*
	DTaP or DT		6 weeks		4 weeks		4 weeks		6 months
	Hib (primary series)
	HbOC		6 weeks		4 weeks		4 weeks
	PRP-T		6 weeks		4 weeks		4 weeks
	PRP-OMP		6 weeks		4 weeks
	IPV		6 weeks		4 weeks		4 weeks		4 weeks
	MMR		12 months*		4 weeks
	Hepatitis B		Birth		4 weeks		8 weeks
	Varicella		12 months		4 weeks
	Pneumococcal conjugate vaccine		6 weeks		4 weeks		4 weeks		8 weeks
	Abbreviated forms used in table: Hib - Haemophilus influenzae type b; IPV - inactivated poliovirus; MMR - measles, mumps, and rubella.     *  These minimum acceptable ages and intervals might not correspond with the optimal recommended ages and intervals for vaccination. The childhood immunization schedule is published each January in the Morbidity and Mortality Weekly Report.       Diphtheria and tetanus toxoids and acellular pertussis vaccine.       The booster dose of Hib vaccine recommended following the primary vaccination series should be administered no earlier than 12 months of age and at least 8 weeks after the previous dose.       It is preferable to administer the fourth dose of the polio series 3 to 4 years after the third dose.     **  Although the age for measles vaccination may be as young as 6 months in outbreak areas where cases are occurring in infants younger than 12 months of age, infants initially vaccinated before the first birthday should be revaccinated at 12 to 15 months of age and an additional dose of vaccine should be administered at the time of school entry or according to local policy. Doses of MMR or other measles-containing vaccines should be separated by at least 4 weeks.       This final dose is recommended at least 4 months after the first dose and no earlier than 6 months of age.       The fourth (booster) dose should be no earlier than 12 months of age and at least 8 weeks after the previous dose.

Yellow Fever Vaccine
Because infants are at high risk of developing encephalitis from yellow fever vaccine, the recommendations for vaccinating infants should be considered on an individual basis. Although the incidence of these adverse events has not been clearly defined, 14 of 18 reported cases of post-vaccination encephalitis were in infants younger than 4 months of age. One fatal case confirmed by viral isolation was in a 4-year-old child. The ACIP and the World Health Organization recommend that yellow fever vaccine never be given to infants younger than 4 months of age. Yellow fever vaccine can be given to infants and children 9 months of age or older if they are traveling to or living in areas of South America and Africa where yellow fever infection is officially reported (see Summary of Health Information for International Travel, also known as the "Blue Sheet") or to countries that require yellow fever immunization (see Comprehensive Yellow Fever Vaccination Requirements). Infants and children 9 months of age or older also should be immunized if they travel outside urban areas within the yellow fever endemic zone (Comprehensive Yellow Fever Vaccination Requirements) and maps in Travelers' Health Information on Yellow Fever). Infants 6 through 8 months of age should be vaccinated only if they travel to areas of ongoing epidemic yellow fever and a high level of protection against mosquito bites is not possible. Immunization of children 4 through 6 months of age should be considered only under unusual circumstances (consult the Centers for Disease Control and Prevention [CDC]), and in no instance should infants younger than 4 months of age receive yellow fever vaccine. Travelers with infants younger than 9 months of age should be strongly advised against traveling to areas with epidemic yellow fever.

Hepatitis A Vaccine or Immune Globulin for Hepatitis A
Infants and children traveling to developing countries are at increased risk of acquiring hepatitis A virus (HAV) infection, especially if their travel is outside usual tourist routes, if they will be eating food or drinking water in settings of questionable sanitation, or if they will be in contact with local residents in settings of poor sanitation (see Travelers' Health Information on Hepatitis A). Although HAV is rarely severe in infants and children younger than 5 years of age, those infected efficiently transmit infection to other infants and children and to adults. Immune globulin (IG) should be given to infants younger than 2 years of age in the same schedule as that recommended for adults (Table 3-6). Children 2 years of age or older should receive the pediatric formulation of HAV vaccine (Tables 3-4 and 3-5) or IG (Table 3-6). The first dose of vaccine should be given at least 4 weeks prior to travel.

Other Diseases
See Travelers' Health Information on Malaria and Travelers' Diarrhea for discussion of malaria and diarrhea in infants.