Immunizations for pediatric transplant patients

Infection remains a major cause of morbidity and mortality in transplant patients. Many infections, however, can be successfully prevented by immunization. This presentation reviews the problems associated with, and the questions that arise concerning the use of routine pediatric vaccines, such as diphtheria-pertussis-tetanus (DPT) and measles-mumps-rubella (MMR). It also reviews the use of special vaccines such as hepatitis B, pneumococcal, and influenza vaccines in transplant patients. Data concerning the use of two experimental, live-attenuated virus vaccines, against cytomegalovirus (CMV) and varicella, are discussed. The live-attenuated varicella vaccine can be predicted to decrease the morbidity and mortality of varicella-zoster virus infection in transplant patients. It has already been given successfully to immunocompromised children and is highly effective in the prevention of varicella.

Varicella vaccine: the American experience

Live attenuated varicella vaccine is safe and effective in preventing chickenpox. The best immune responses occur in healthy children. Leukemic children have a 50% incidence of mild-to-moderate adverse effects but have a high degree of protection once immune reactions to varicella-zoster virus (VZV) have developed. Adult vaccinees have a lower degree of protection (70%) than children. Vaccinees who develop breakthrough varicella usually have a modified infection. Another significant advantage of vaccination is that in leukemic children it leads to a lower incidence of zoster than after natural chickenpox. It is possible to differentiate between vaccine-type and wild-type VZV using a combination of polymerase chain reaction and restriction endonuclease digestion. A new assay for antibodies to VZV measured by latex agglutination reveals that 8-10 years after vaccination antibodies are detectable in greater than 90% of leukemic children who have not had breakthrough varicella.

A controlled trial of acyclovir for chickenpox in normal children

BACKGROUND

Chickenpox, the primary infection caused by the varicella-zoster virus, affects more than 3 million children a year in the United States. Although usually self-limited, chickenpox can cause prolonged discomfort and is associated with infrequent but serious complications.

METHODS

To evaluate the effectiveness of acyclovir for the treatment of chickenpox, we conducted a multicenter, double-blind, placebo-controlled study involving 815 healthy children 2 to 12 years old who contracted chickenpox. Treatment with acyclovir was begun within the first 24 hours of rash and was administered by the oral route in a dose of 20 mg per kilogram of body weight four times daily for five days.

RESULTS

The children treated with acyclovir had fewer varicella lesions than those given placebo (mean number, 294 vs 347; P less than 0.001), and a smaller proportion of them had more than 500 lesions (21 percent, as compared with 38 percent with placebo; P less than 0.001). In over 95 percent of the recipients of acyclovir no new lesions formed after day 3, whereas new lesions were forming in 20 percent of the placebo recipients on day 6 or later. The recipients of acyclovir also had accelerated progression to the crusted and healed stages, less itching, and fewer residual lesions after 28 days. In the children treated with acyclovir the duration of fever and constitutional symptoms was limited to three to four days, whereas in 20 percent of the children given placebo illness lasted more than four days. There was no significant difference between groups in the distribution of 11 disease complications (10 bacterial skin infections and 1 case of transient cerebellar ataxia). Acyclovir was well tolerated, and there was no significant difference between groups in the titers of antibodies against varicella-zoster virus.

CONCLUSIONS

Acyclovir is a safe treatment that reduces the duration and severity of chickenpox in normal children when therapy is initiated during the first 24 hours of rash. Whether treatment with acyclovir can reduce the rare, serious complications of chickenpox remains uncertain.

The incidence of zoster after immunization with live attenuated varicella vaccine. A study in children with leukemia. Varicella Vaccine Collaborative Study Group

BACKGROUND

The Oka strain of live attenuated varicella vaccine is immunogenic and highly protective, but there has been concern about the risk of zoster after immunization.

METHODS

We examined the incidence of zoster, risk factors for it, and measures of immune response in children with leukemia who received the vaccine and in appropriate controls.

RESULTS

After a mean follow-up of 4.1 years, zoster was documented in 13 of the 548 vaccinated children with leukemia (2.4 percent). In a subgroup of 96 vaccinated children matched prospectively with 96 children with leukemia who had had natural varicella infections, there were 4 cases of zoster among the vaccinated children and 15 among the controls, for crude incidence rates of 0.80 and 2.46 cases per 100 person-years, respectively (P = 0.01). Of the total of 13 vaccinated children who had zoster, 11 had a skin rash due to varicella-zoster virus, either from the vaccine itself or from breakthrough varicella after household exposure in the period between immunization and the documentation of zoster. In the 268 children who had any type of rash caused by varicella-zoster virus after vaccination, as compared with those who did not have a rash, the relative risk of subsequent zoster was 5.75. For the 21 vaccinated children who received bone marrow transplants, as compared with those who did not, the relative risk of zoster was 7.5. Cell-mediated immunity as assessed by lymphocyte stimulation was lower in 4 children in whom zoster later developed than in 29 controls who had been vaccinated but who did not have zoster (mean stimulation index, 5.1 vs. 23.8; P = 0.0001).

CONCLUSIONS

In children with leukemia who receive the live attenuated varicella vaccine, the subsequent incidence of zoster is lower than in children who have natural varicella infections.

Human immune responses to live attenuated varicella vaccine

Experiences with the use of the live attenuated varicella vaccine for the immunization of varicella-susceptible adults and healthy and leukemic children are described. Healthy children, for whom a greater than 90% rate of seroconversion and protective efficacy was noted, showed the best response. Leukemic children experienced a 40%-50% rate of vaccine-associated rash; protective efficacy approached 90%. For adults, a 10% rate of vaccine-associated rash and a protective efficacy of 70% were noted. Breakthrough cases of vaccine were mild, indicating modification of the course of the illness by the vaccine.

Measurement of antibodies to varicella-zoster virus by using a latex agglutination test

We evaluated a rapid, simple-to-perform assay for detecting antibodies to varicella-zoster virus by latex agglutination (LA); dilutions of test sera were added to latex particles coated with varicella-zoster virus antigen. We tested 878 serum specimens by LA and with fluorescent antibody to membrane antigen; of these, 227 were also tested by a commercially marketed enzyme-linked immunosorbent assay (ELISA). LA was almost as sensitive as the fluorescent antibody to membrane antigen test and more sensitive than ELISA. No cross-reacting antibodies were detected by LA, and false-positive reactions were rare.

Immunization practices in children

The resurgence of measles has highlighted concerns about U.S. programs for immunization in infants and children. In order to put the problems into perspective, this review will address such issues as the safety of pertussis vaccines; oral vs inactivated poliovirus vaccine; vaccines for measles-mumps-rubella, Hemophilus influenzae type B, and hepatitis B; and varicella vaccine.

Antibodies to varicella-zoster virus glycoproteins I, II, and III in leukemic and healthy children

A dot ELISA was used to analyze the antibody response to varicella-zoster virus glycoproteins I, II, and III in leukemic and healthy children who either developed natural varicella or received the live attenuated varicella vaccine. Both groups of children showed an antibody response to these viral glycoproteins. The antibody response to all three glycoproteins showed excellent persistence over the 3-year period of study. None of the glycoproteins provoked an antibody response that could be shown to correlate with protection from breakthrough varicella after household exposure to chickenpox or from zoster in leukemic vaccinees.

Viral vaccines of the future

Varicella vaccine seems close to licensure in the United States. It is likely to be recommended for routine use in healthy children, either administered singly or as a combination of MMR. Healthy children who have been immunized develop excellent antibody and cell-mediated responses to VZV in the absence of significant adverse effects, and they are well protected against subsequent infection with VZV. Although it will not be known for certain for many years, it seems most unlikely that immunization will result in an increased incidence of zoster, a secondary type of infection with VZV that is caused by reactivation of latent VZV. Varicella vaccine may also be given to healthy adults who have never had chickenpox with a great degree of success, although its protection is somewhat less effective than in children. Leukemic children who are at high risk for developing severe or fatal varicella also derive a great deal of protection from varicella vaccine, but the vaccine must be administered to them with great caution. Other viral vaccines that may be licensed in the future but that are not as fully developed as varicella vaccine include vaccines against CMV, hepatitis A, HSV, and AIDS. These are in various degrees of study, and, should any of them be licensed, it is uncertain whether they would be live attenuated, recombinant, or subunit vaccines. It is hoped, however, that they will eventually be licensed for future use because if effective, they could significantly decrease morbidity and mortality of infants, children, and adults.

Live attenuated varicella vaccine: protection in healthy adults compared with leukemic children. National Institute of Allergy and Infectious Diseases Varicella Vaccine Collaborative Study Group

Protection against varicella infection was assessed in leukemic children and healthy young adults who were immunized with live attenuated varicella vaccine. Attack rates of breakthrough infection following household exposure to varicella in 102 children and 26 adults were similar whether one or two doses of vaccine had been given. The mild breakthrough illness was also similar after one or more doses. Specific antibody titers were similar 1 year after immunization whether individuals had received one or two doses. Humoral and cell-mediated immunity to varicella-zoster virus (VZV) was lower in these vaccinees than in persons who had experienced natural varicella infection. Protection after natural infection in adult family members exposed to varicella was superior to that in vaccinees; none developed varicella infection. These observations suggest that immunization induces less protection than does natural disease in leukemic children and young adults. This may be partly due to the nature of the vaccine virus, but because responses of adults were similar to those of leukemic children, it suggests also that both of these groups have impaired immune responses to VZV. Boosting of humoral immunity after exposure to VZV was common and was observed in healthy adults with past natural infection and in vaccinated adults and leukemic children.

Prospects for use of a varicella vaccine in adults

Five to 10% of people reach adulthood still susceptible to VZV, which generally causes more severe primary disease in adults than seen in children. A live attenuated varicella vaccine was developed in Japan in the early 1970s and has now been tested in several trials in healthy children, immunocompromised children, and healthy adults. The vaccine is highly immunogenic in healthy children, conferring immunity for at least 7 to 10 years with a high degree of protective efficacy. In several trial in adults, the vaccine has been shown to be highly immunogenic. Humoral and cell-mediated immunity wanes in vaccinated adults, however, with antibodies to VZV detectable in approximately only 80% of vaccinated individuals after 1 year, and in approximately 70% from 2 to 6 years after vaccination. In leukemic children who have been vaccinated, however, this loss of detectable antibody has not been correlated with a reduction in protection during this time. Similar analyses have not been made in vaccinated adults because the numbers intimately exposed to VZV have been small. Protective efficacy after household exposure is approximately 65% in adults; however, when breakthrough (i.e., in those who have seroconverted) illness has occurred, it has invariably been mild, so that efficacy in preventing severe disease has been 100%. "Vaccine failures," however, may develop full-blown varicella. Side effects of immunization in adults are mild: a transient local reaction occurs in 10 to 21% and a mild rash in 6 to 8%. There is a theoretical risk of transmission of the attenuated virus if a vaccine-induced rash occurs, which has been documented only in contacts of vaccinated leukemics (any secondary disease has also been mild). To date, there has been no evidence that vaccination increases the risk of developing zoster; on the contrary, studies in leukemic children, who may be considered a "sentinel population" in this regard, suggest that the risk of zoster after vaccination may be reduced compared with the risk after natural infection. Susceptible adults who would most benefit from vaccination against VZV include health care workers, those who care for small children, women of child-bearing age prior to pregnancy, military recruits, and college students.

Live attenuated varicella vaccine: evidence that the virus is attenuated and the importance of skin lesions in transmission of varicella-zoster virus. National Institute of Allergy and Infectious Diseases Varicella Vaccine Collaborative Study Group

To examine whether the live varicella vaccine virus is attenuated, we analyzed varicella vaccine-induced contact cases of clinical chickenpox in healthy siblings of immunized children with leukemia. A rash developed approximately 1 month later in 156 children with leukemia who had been vaccinated. Vaccine-type virus was isolated from 25 of these children. Of 88 known susceptible healthy siblings who were exposed to a vaccine with a rash and from whom follow-up information was available, there was evidence of infection in 15 (17%). Of 15 siblings with seroconversion, 11 (73%) also acquired a mild rash with an average of 38 lesions and no accompanying systemic symptoms. Vaccine-type virus was isolated from four of the contact siblings. Tertiary transmission was documented once. Contact siblings with seroconversion were protected during future household exposure to chickenpox, which occurred in four instances. There was a direct relationship between transmission from vaccinees to varicella-susceptible close contacts and the presence and number of skin lesions in children with leukemia after vaccination. We conclude that in the transmission of varicella, the virus probably originates from skin lesions of infected persons and reaches the respiratory tract of those with secondary cases by the airborne route. On the basis of the mildness of the contact illness, the higher-than-normal rate of subclinical primary infection with varicella-zoster virus in contacts, and the lower-than-normal rate of spread of the vaccine virus to susceptible children in the household, we further conclude that the vaccine virus is attenuated. There was no evidence of reversion of the vaccine virus to virulence.

Varicella-zoster virus glycoprotein oligosaccharides are phosphorylated during posttranslational maturation

Varicella-zoster virus (VZV)-infected human embryonic lung fibroblasts (HELF) do not release infectious virions into their growth medium. Extracellular virions are pleomorphic, suggesting that they are partially degraded before their release from cells. To examine the intracellular pathway of viral maturation, [2-3H]mannose-labeled virus-encoded glycoproteins were isolated from VZV-infected HELF. Oligosaccharides attached to the glycoproteins were processed to complex-type units, some of which were phosphorylated. The major intracellular site of accumulation of VZV gpI was found to be perinuclear and to correspond to that of the cation-independent mannose 6-phosphate (Man 6-P) receptor. Subsets of VZV-containing cytoplasmic vacuoles were coated, Golgi-associated, or accessible to endocytic tracers. Phosphorylated monosaccharides protected HELF from the cytopathic effect of VZV in proportion to their ability to block Man 6-P receptor-mediated endocytosis. These data suggest that the unusual phosphorylated oligosaccharides mediate an interaction between VZV and Man 6-P receptors of the host cell; this interaction may be responsible for withdrawal of newly synthesized virions from the secretory pathway and for their diversion to prelysosomal structures.

Varicella-zoster virus infections in children infected with human immunodeficiency virus

Primary varicella-zoster (VZ) infection in eight children with perinatally acquired human immunodeficiency virus infection tended to be severe, prolonged, complicated by bacterial infections and in one case fatal. Depletion of CD4-lymphocytes was associated with chronic and recurrent VZ infection. In some patients convalescent VZ antibody titers were low and did not correlate with recurrence of VZ lesions. Administration of acyclovir appeared to be beneficial in suppressing VZ in human immunodeficiency virus-infected children with primary or recurrent VZ infection.

Herpes zoster in an adult recipient of live attenuated varicella vaccine

A healthy 30-y-old female physician who was immunized with two doses of live attenuated varicella vaccine developed a localized case of zoster involving the right T8-10 dermatomes 36 mo after vaccination. The virus isolated from her rash was an unusual wild-type of varicella-zoster virus. After immunization she developed detectable antibodies to varicella-zoster virus, but antibodies were no longer detectable after 20 mo. Six months before development of zoster, she was exposed to a patient with varicella; however, she did not develop a clinical illness although she again became seropositive. To date, this is the only reported case of zoster among 187 healthy adult vaccinees.

Persistence of immunity to varicella in children with leukemia immunized with live attenuated varicella vaccine

To determine the safety and efficacy of varicella vaccine, we studied 437 children in remission from leukemia who were immunized with live attenuated varicella virus. Three hundred one of the patients received two doses of vaccine and 136 received a single dose of vaccine from 1 of 10 lots from two manufacturers. The patients have been followed for an average of three years (range, one to six). Seroconversion occurred in 88 percent of the 437 children after the first dose of vaccine and in 98 percent after one or two doses. The proportions of patients who were seronegative after one, three, and five years were 20, 25, and 30 percent, respectively, with little change over time in the geometric mean titers of specific antibody (6.3, 6.5, and 5.7, respectively). Chickenpox has been documented in 36 vaccinated patients (8 percent) who had 3 to 640 vesicles (mean, 100), mild illness, and no complications. Of the 83 vaccinated patients exposed to varicella within their families, 11 had chickenpox; the attack rate was 14 percent (8 percent among seropositive patients, 29 percent among seronegative patients). There was no relation between the time since vaccination and either the attack rate or the severity of the breakthrough illness. Two doses of vaccine appeared to be no more effective than a single dose. Of the 372 patients receiving maintenance chemotherapy when immunized, 149 (40 percent) had a rash, which was treated with acyclovir in 16 children (4 percent) and became a severe febrile illness in 4. These reactions were not fatal and were all associated with vaccine lots, the use of which has since been discontinued. We conclude that in children in remission from leukemia, varicella vaccine is safe and induces an immunity to chickenpox that persists for more than three years.

Immunization of healthy adults with live attenuated varicella vaccine

Live attenuated varicella vaccine was administered to healthy varicella-susceptible adults. Of 187 adults immunized with the Oka strain of vaccine, seroconversion to varicella-zoster virus (VZV) occurred in 82% after one dose and in 94% after two doses. Adverse effects were unusual. After immunization, one subject developed mild zoster caused by wild-type virus. Twelve adults developed a mild breakthrough case of chickenpox after exposure to VZV. Protection after household exposure was observed in nine (56%) of 16; however, the illness in all seven patients with breakthrough illness was modified, with an average of only 24 vesicles. Subjects seropositive at household exposure were unlikely to develop a breakthrough illness. Approximately 25% of vaccinees who seroconverted lost detectable antibodies to VZV after vaccination, but even those who became seronegative were partially protected. Varicella vaccine offered significant protection against severe chickenpox in healthy adults.

Western blot analysis of antibody to varicella-zoster virus

We used western blot (WB) to compare the IgG response to varicella-zoster virus (VZV) after chickenpox (CP), zoster, and administration of a live attenuated varicella vaccine (LAVV). After CP, 13 of 14 normal children had antibody to glycoprotein (gp) I (99 and 92 kilodaltons [kDa]), 11 had antibody to gpII (133 kDa), and 10 had antibody to gpIII (119 kDa). Bands at 150 and 35 kDa were also seen in 13 and 11 sera, respectively. Bands to gpI, gpII, and p35 were more intense after zoster than CP. After one dose of LAVV, eight of eight normal children had gpI antibody. In leukemic children, gpI antibody appeared in 18 (56%) after one dose and in 25 (89%) after two doses. Upon household exposure, leukemic vaccinees who developed CP were less likely than those protected to have prior gpIII and p35 antibodies. As seen after zoster, WBs after breakthrough CP showed intense responses to VZV antigens. Thus, WB helps distinguish secondary from primary antibody responses to VZV.

The risk of zoster after varicella vaccination in children with leukemia

We examined the incidence of zoster in 346 children with underlying acute lymphoblastic leukemia who were immunized with live attenuated varicella vaccine while in remission. We also compared a subset of 84 of these children with a matched group of 84 children with leukemia who had had natural infection with varicella. Of the 346 vaccinated children, 5 (1.45 percent) became infected with zoster after 10,878 months of observation, for an incidence of 0.552 case per 100 person-years. Among the matched pairs of subjects, zoster occurred in 3 (3.6 percent) of the 84 vaccinated subjects during 2936 months of observation--an incidence of 1.23 cases per 100 person-years--and in 11 (13.1 percent) of the subjects with natural infection during 4245 months--an incidence of 3.11 cases of zoster per 100 person-years. Although the incidence of zoster was more than twice as high in the control children as in the vaccinated children (3.11 vs. 1.23 cases per 100 person-years), a Kaplan-Meier product-limit analysis revealed no significant differences in incidence between the two groups. Children from both groups in whom leukemia recurred were more likely to contract zoster than those who did not have a recurrence (7 of 35 vs. 7 of 133, P less than 0.025). Zoster was not a marker for impending relapse. No case of zoster was severe or disseminated. We conclude that the incidence of zoster following immunization with live attenuated varicella vaccine is no greater than that following natural varicella infection.