Rotavirus-induced acute gastroenteritis is one of the last great universal infections of childhood that has yet to be controlled by improved hygiene or a vaccine. Because of the severity of the associated dehydration, it is the greatest killer of infants and young children in developing countries. Rotavirus is also a disease of enormous effect, sometimes unappreciated, in die developed world.
The next vaccines for rotavirus soon will be licensed, more than 30 years after its identification as a human pathogen. It will be imperative for pediatricians to have a thorough understanding of rotavirus disease so they can explain to parents the necessity for a rotavirus vaccine.
THE DISEASE: SYMPTOMS, PATHOGENESIS, AND TREATMENT
Symptoms of rotavirus disease are those classic for infant and childhood gastroenteritis, including diarrhea, vomiting, and fever, with vomiting being especially prominent. In one study, vomiting was observed in 96% of rotavirus patients, compared with 58% in patients with diarrhea caused by other pathogens. ' Scandinavian researcher Karston Hjelt2 stated vomiting was the single most characteristic symptom of rotavirus disease. Indeed, case definitions of rotavirus disease used in vaccine clinical trials include forceful vomiting within 24 hours of the onset of symptoms.3
Vomiting commonly begins 1 or 2 days before diarrhea, often limiting the effectiveness of oral-rehydration therapy. Fever, which invariably accompanies rotavirus infection, often is greater than 1020F and typically lasts 2 to 4 days. Diarrhea lasts 2 to 3 days longer than that found in infants with nonrotavirus diarrhea. As a consequence, infants hospitalized for rotavirus disease tend to be retained longer than those hospitalized with diarrhea of other causes.
Figure 1 . (A) Normal intestinal villus with intact epithelial layer. (B) Marked disruption and vacuolization of villous epithelial cells during rotavirus infection.
Vomiting, fever, and diarrhea combine to cause severe dehydration, which explains why the disease is particularly lethal in both the developed and developing world. Deaths that occur in the developed world can usually be attributed to the rapidity with which dehydration becomes irreversible, even when medical care is sought early in the course of the illness. In a classic study of 21 rotavirus deaths observed in Toronto, Ontario, Canada, 10 infants were dead upon arrival at the hospital, and 10 were hospitalized too late to respond to treatment.4 The average age was approximately 1 . Of the 21 infants, 19 were normal, healthy infants before the rotavirus infection, all had prominent vomiting, and 15 died less than 24 hours after onset of symptoms.
The pathogenesis of rotavirus diarrhea is understood mainly from animal model studies and involves several concomitant processes. Disease mechanisms appear to be consistent in all mammalian species.
Histopathological changes are limited to the lumen of the small intestine, beginning in the duodenum and progressing distally to the ileum.5 The earliest and principal histopathological lesion is variolation of the apical epithelium of the microvilli (where numerous virions are present intracellularly), proceeding to rapid cell death accompanied by denuding and blunting of the microvilli (Figure 1). The lack of functional intestinal villous epithelial cells disrupts normal fluid and electrolyte control by the intestine, leading to diarrhea. In hosts that recover, normal intestinal villi regenerate within a few days.
Other factors have been proposed to contribute to the severity of rotavirus disease. Foremost is the activity of a nonstructural rotaviral protein, described as the first viral enterotoxin, which destabilizes cell membranes, including the endoplasmic reticulum, and disrupts normal transport of serum calcium.6 It has also been suggested that rotavirus-induced activation of the enteric nervous system leads to increased peristalsis and disruption of fluid homeostasis.7 There is no convincing evidence that rotavirus consistently infects or causes symptoms in sites other than the intestine.
Oral rehydration therapy using either glucose or rice-based solutions fortified with potassium, sodium, and citrate salts may be the only treatment required for mild and moderate dehydration. The World Health Organization has distributed large quantities of packets of drysalt combinations worldwide with the intention that mothers would successfully treat their infants at extremely low cost.8 For a variety of reasons, both practical and sociological, this program has had less effect on developing countries than hoped for, reinforcing the belief that only an effective vaccine is likely to provide consistent protection.
Oral rehydration therapy is also the initial treatment for mild to moderate dehydration in developed countries; intravenous hydration is reserved for children who are either vomiting too vigorously or are too moribund to be fed an oral preparation safely. Once a child is successfully rehydrated by parenteral treatment, recovery is normally rapid and fatality is extremely rare. A combination of lack of understanding by parents of the necessity for rehydration and a paucity of available facilities to provide it contributes to the massive rotavirus mortality in developing countries.
In crowded situations favorable to spread of the virus, such as daycare centers and hospital nurseries, stringent hygienic measures are essential. Infants with rotavirus disease routinely shed 1010 to 10" infectious particles per gram of feces,9 Etiologic diagnosis of rotavirus is often thought unnecessary in both developed and developing countries because treatment of dehydration varies little regardless of cause.
Etiologic diagnosis of rotavirus is often thought unnecessary in both developed and developing countries because treatment of dehydration varies little regardless of cause.
while the human infectious dose may be as low as 10' infectious particles.10 In places of prolonged close contact of infants, it is essential that rigorous handwashing procedures are used and mat exposed surfaces are moroughly washed with a disinfectant such as 95% ethanol.
SUSCEPTIBLE POPULATIONS AND DISEASE EPIDEMIOLOGY
The Developed Nations
Rotavirus infects virtually all infants and young children by 2 to 3 years of age.1 ' In a prospective study in a Virginia suburb, 88% of these first infections were symptomatic;'2 first infections are often moderate to severe and occasionally fatal. Rotavirus disease is usually concentrated within me age range of 3 to 24 months, although cases in newborn infants, older children, and adults (including elderly adults) also occur and may be clinically significant.
Spread of rotavirus infection from affected infants to their siblings and parents is very common. In a prospective study involving rotavirus infection in 18 Canadian hospitals, diarrhea occurred shortly after the index case in 74% of household contacts younger than age 3 and in 29% of those age 18 or older.13 Presumably because adults retain immunity from repeated infant and childhood infections and can better tolerate dehydration than infants, disease in parents is usually not as severe. Nevertheless, rotavirus disease in parents of infected children is often serious enough to dictate absenteeism from work. However, the cost of lost work by parents who are themselves ill is seldom included in die computation of the total societal cost of rotavirus disease.
Some recent reports indicate neonates and very young infants, originally thought to be effectively protected by either transfer of maternal immunity or immaturity of die small intestine, may also often experience severe disease. This is especially true of nosocomial infections14 but also occurs in community-acquired disease.1415
Etiologic diagnosis of rotavirus is often thought unnecessary in bodi developed and developing countries because treatment of dehydration varies little regardless of cause. In me United States and otfier developed countries, most infants experience one to tfiree attacks of diarrhea per year, and at least 50% of nonbacterial diarrhea occurring in me winter is caused by rotavirus. The gready heightened winter activity of most pediatricians' telephone logs associated with reports of vomiting and diarrhea attests to the economic effect of rotavirus on a pediatric practice. A survey in 2003 in two suburban and two urban pediatric practices and one urban hospital emergency department in the Philadelphia area revealed mat reports of gastroenteritis accounted for 2,199 telephone contacts and 922 office visits (800 of which were to die private practices) in a single season (Coffin S, oral communication, February 2004).
Recent national surveys indicate between 50,000 and 100,000 infants are hospitalized yearly in die United States, and 20 to 40 rotavirus deaths occur.16 The medical and societal costs of this disease are estimated at well over $1 billion per year in the United States; similar disease burdens have been reported in western Europe. Clearly, a rotavirus vaccine that reduces this disease burden should have universal application.
In developing nations where poor hygiene, malnourishment, crowding, and other contributing factors favor transmission of all diseases, infants have as many as 10 or more attacks of gastroenteritis per year. Furthermore, tropical countries may experience rotavirus disease year-round. In developing nations, rotavirus disease is distinguished from other causes of gastroenteritis by its exceptional severity, particularly with regard to dehydration. Despite me fact that many more cases of gastroenteritis occur per infant per year in developing nations, rotavirus causes about 50% of all cases requiring hospitalization in both developed and developing nations.17
Figure 2. Two rotavirus surface proteins, vp4 (P type) and vp7 (G type), determine rotavirus serotype.
Unlike the developed world, where even severe rotavirus dehydration usually can be successfully treated, the outcome of rotavirus disease is often more dire in the developing world. Mothers accustomed to observing frequent diarrhea and vomiting in their infants may not recognize unusually severe dehydration or may not be sensitized to its grave consequences. When medical assistance is sought, it may be too late because travel to treatment centers may be impossible or medical facilities mat exist may lack the skills to administer lifesaving treatment. It is estimated mat as many as 600,000 rotavirus deaths occur each year; about 2,000 babies die every day from rotavirus.18 A vaccine would be the ideal approach to reduce deaths from rotavirus disease effectively.
ROTAVIRUS CHARACTERISTICS: STRUCTURE AND SEROTYPE
Rotavirus is named because of the wheel- or rota-like appearance of mature virus particles. The structure of the virus consists of three concentric layers, each with a distinct structural and antigenic composition.19 The outer shell of the virus is made up of a coat protein termed viral protein (vp) 7 and a spike protein termed vp4 (Figure 2, see page xxx). Both proteins evoke antibodies that neutralize virus infectivity (ie, determine viral serotype); serotypes based on mese two proteins are termed G type (for the glycoprotein vp7) and P type (for the protease-sensitive spike protein vp4). Therefore, rotaviruses are similar to the influenza viruses, where two viral surface proteins, the viral haemaglutinin (H) and neuraminidase (N), determine serotype. Influenza serotypes are described on the basis of diese two surface proteins (eg, HlNl), as are rotaviruses (eg, PIaGl).
Worldwide, the most common G type is Gl, but in certain seasons and specific geographic areas, other strains may predominate, including strains G2, G3, G4, G8, and G9. The most common P type is PIa. Strain PIaGl is the most common serotype worldwide, but numerous other combinations of P and G serotypes occur.
GOALS FOR ROTAVIRUSVACCINE
A reasonable expectation for a rotavirus vaccine would be one that does not cause adverse events such as low-grade fever, intussusception, mild vomiting or diarrhea, or systemic symptoms such as poor feeding and irritability. The vaccine should be highly effective (greater than 85%) at preventing moderate-to-severe gastroenteritis and significantly reduce calls and visits to pediatricians' offices and visits to emergency departments. A rotavirus vaccine should also dramatically reduce or eliminate hospitalizations and deams caused by the disease.
EARLY ATTEMPTS AT HUMAN VACCINATION: ANIMAL ROTAVIRUSES
Although human rotavirus was first identified in 1973, means of propagating human rotavirus were not described until a decade later.20 However, bovine rotavirus grew easily in cell culture. Clinical trials in Finland of a bovine rotavirus vaccine (strain RIT 4237) found that the vaccine was safe and caused greater than 80% protection against "clinically significant disease."21 However, in subsequent trials in Africa and elsewhere, bovine rotavirus vaccine showed little protection. Although the bovine vaccine was discontinued, these trials established mat a bovine rotavirus was safe and immunogenic in infants.
A second attempt at use of a bovine vaccine was pursued wim a strain named WC3, isolated in Pennsylvania. Strain WC3 was safe and protective in clinical trials performed in Philadelphia but ineffective in later trials in Cincinnati, OH, and in Africa.22 Similar to the first bovine vaccine, it caused no adverse events and was immunogenic in infants.
Investigators at the National Institutes of Health pursued studies with a rotavirus isolated from a rhesus monkey - strain RRV, for rhesus rotavirus. RRV sometimes caused fever and mild symptoms of gastroenteritis. Two efficacy trials in Finland showed protection levels of 67% to 80% against severe gastroenteritis.23 Another trial in Venezuela showed promising protection against disease,23 but several ineffective trials in the United States led to discontinuation of RRV studies.24
MULTIVALENT ROTAVIRUS REASSORTANT VACCINES
The next approach was to make combination, or reassortant, rotaviruses between animal and human rotavirus strains. Reassortant rotavirus vaccine candidates were selected to contain animal virus genes that attenuated the virus for growth in humans and genes that coded for human rotavirus proteins that evoked virus-neutralizing antibodies. Candidates for reassortant rotavirus vaccine were constructed by taking advantage of the segmented genome of rotavirus; following a mixed cell-culture infection of one animal and one human strain, gene segments from both strains were often packaged in the same virion.
Reassortants of Strain RRV
A tetravalent vaccine of RRV reassortants (designated Rotashield) was constructed of single-gene reassortants coding for the human antigens Gl, G2, and G4. RRV in its native form was added because it was similar to human G3 rotavirus. The vaccine was administered orally in three doses with the last dose completed before age 6 months because the vaccine was more reactogenic in older infants.
Similar to the RRV vaccine, Rotashield caused fevers in about 20% of recipients and, less commonly, other signs of gastroenteritis. Efficacy of Rotashield in 6,000 infants in four trials in the United States, Finland, and Venezuela was approximately 50% to 70% against all rotavirus disease and 70% to 90% against severe disease.24
In August 1998, Rotashield was licensed and recommended for use in the United States.25·26 By July 1999, however, 15 cases of intussusception following the use of Rotashield vaccine were reported to the Vaccine Adverse Events Reporting System.27 As a result, the use of the vaccine was temporarily suspended pending an investigation to determine whether the relationship between vaccination and intussusception was causal or coincidental.27·28
Subsequent studies by the Centers for Disease Control and Prevention found the association between the Rotashield vaccine and intussusception was "strong, temporal, and specific."29 The relative risk for intussusception 3 to 14 days after administration of the first dose of Rotashield was 21.7 (P < .001). The attributable risk for intussusception following Rotashield vaccine was estimated to be approximately one case for every 10,000 vaccinated children.29·30 The recommendation to use Rotashield was withdrawn in October 1999.31 Because intussusception is apparently not a consequence of natural rotavirus infection, the mechanism of intussusception caused by Rotashield remains unclear.
Reassortants of Bovine Rotavirus Strain WC3
A multivalent reassortant vaccine based on a bovine WC3 rotavirus genome was constructed similar to Rotashield, except that unlike RRV, WC3 was not associated with adverse events and WC3 alone was not included in the vaccine. Taking advantage of the combined benefits of three doses, and of combining PIa and Gl reassortants, a multicenter efficacy trial was conducted in which 439 subjects in 10 cities were given a quadrivalent vaccine composed of a WC3 backbone and human rotavirus genes coding for Gl, G2, G3, and PIa reassortants.32 No excess adverse events were observed in vaccines; vaccine efficacy was 75% against all rotavirus disease and 100% against severe disease.
Later, a G4 reassortant of WC3 was added to assemble a pentavalent rotavirus vaccine (PRV) containing Gl, G2, G3, G4, and PIa. To determine the ideal dose of PRV, a dose-ranging study was conducted in 1,349 infants in Finland given three doses of a high, medium, or low concentration of virus.33 No adverse events were observed, and efficacy data indicated that all concentrations of PRV were similarly protective against rotavirus disease.
Although WC3 -reassortant rotavirus vaccine did not cause adverse events, the total number of subjects in these combined trials was far less than would be required to detect a very rare sequella occurring at the incidence at which intussusception was associated with Rotashield. Therefore, the pentavalent bovine-human reassortant rotavirus vaccine - now called Rotateq - is being evaluated in a double-blinded, placebocontrolled, international study of more than 60,000 infants. This trial, the single largest clinical trial ever sponsored by a pharmaceutical company, will determine whether Rotateq meets acceptable safety standards with regard to association with intussusception.
Although the study is still in progress and blinded, preliminary data are encouraging. The total number of cases of intussusception is fewer than expected, and those cases are not clustered within a 1or 2-week period after administration of the first or second dose of vaccine or placebo (Heaton PM, oral communication, April 2004). It is anticipated that this trial will be completed during 2004. If it is proven safe, a product license application for Rotateq will be filed with the Food and Drug Administration.
MONOVALENT ATTENUATED HUMAN ROTAVIRUS VACCINE
The other vaccine in advanced clinical trials consists of a human rotavirus attenuated by cell-culture passage. In 1989, the PIaGl rotavirus strain was isolated from an infant with gastroenteritis and weakened by serial passage in monkey kidney cells, similar to the strategy used to make the current measles, mumps, rubella, and varicella vaccines.34
In a US efficacy study, protection was compared between 2 1 3 infants given two doses of vaccine and an identical number of placebo recipients.34 A 14% excess of fevers was observed 3 and 4 days after administration of vaccine. The vaccine provided 89% protection against all rotavirus disease, 78% protection against moderate disease, and 100% protection against very severe disease.
To further attenuate this virus, researchers subjected the vaccine virus to additional passages in monkey kidney cells (Ward RL, oral communication, April 2004). The resulting virus was renamed Rotarix and is undergoing extensive clinical trials in developed and developing countries. In initial studies in Finland, it was reported that high doses of vaccine caused no adverse events, and 73% to 96% of infants developed a virusspecific immune response.35 Preliminary efficacy studies of Rotarix performed in Venezuela, Mexico, and Brazil found it was 83% to 91% protective against moderate to severe disease.36
Rotavirus causes severe and often lifethreatening illness. Universal application of a safe and protective vaccine is justified in both developed and developing nations. Two vaccine candidates, one monovalent (Rotarix) and one multivalent (Rotateq), appear to meet these requirements and are likely to be licensed in the United States in the next 2 or 3 years. Both vaccines exhibited similar safety characteristics.
There is little doubt that Rotateq and Rotarix will be shown to be effective for routine protection of infants. Unfortunately, despite numerous clinical trials, me most common serotype (PIaGa) commonly has been encountered as a natural challenge. Therefore, it is not known whether either vaccine possesses advantages in different epidemiological situations.
Continuing the analogy with influenza virus, it may be that optimum protection against different serotypes requires a vaccine that is precisely homologous in antigen composition. If so, Rotateq would provide protection against me most common serotype PIaGl because in includes both PIa and Gl rotavirus reassortants. Further, it would be expected to provide superior protection against G2, G3, and G4 wild-type virus because it contains reassortants of those specificities. In the case of a natural challenge with a serotype that was not Gl , G2, G3, or G4, a Rotateq preparation containing a WC3 reassortant expressing the new G serotype could be formulated readily.
The monotypic Rotarix may provide ideal protection against the PIaGl rotavirus because it is composed solely of the PIaGl strain. It may also provide cross-protection against other rotavirus serotypes adequate to protect against severe and life-threatening disease. In such a case, its monotypic composition may also provide significant economic savings in manufacturing.
The resolution of these questions may have to await extensive post-licensure experience with each vaccine. In the future, possible application of rotavirus vaccine for other situations also should be explored, including use in older children to limit nosocomial infection, use in geriatric populations, use in the immunocompromised host, and possibly use in parents and other adults in contact with infants with rotavirus.
Both Rotarix and Rotateq likely are to be launched at prices beyond those affordable in the poorest and neediest less-developed countries. It is essential that there be vigorous pursuit of new technologies to manufacture these products at drastically reduced cost if their true lifesaving potential is to be achieved.
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