Rotavirus Vaccines Rolled Out
Vaccinologists battle an intestinal virus to prevent one of the leading causes of potentially deadly diarrheal disease in infants
By Kristen Jill Kresge
As a medical epidemiologist at the US Centers for Disease Control and Prevention (CDC) Umesh Parashar has spent the last 10 years of his career chasing an intestinal virus that, outside of medical circles, few people have ever heard of. Even so, rotavirus is a ubiquitous infection among infants and is the most common pathogen associated with the severe diarrheal disease known as acute gastroenteritis that is responsible for around 600,000 deaths and more than two million hospitalizations each year worldwide in children under five years of age.
The death toll in resource-poor countries and soaring medical costs in industrialized nations associated with such a pervasive infection spurred scientists into developing vaccines that could prevent the severe and all too often deadly dehydration caused by this disease, launching a 25 year quest that Parashar refers to as a "rollercoaster ride." Despite early success, the rotavirus vaccine efforts faced a serious setback when the first licensed product was abruptly pulled from the shelves by its manufacturer after safety concerns surfaced in post-marketing surveillance, creating higher hurdles for newer vaccines to clear and ensuring a longer road to approval and licensure.
However the continued efforts by vaccine manufacturers GlaxoSmithKline (GSK) and Merck culminated earlier this year in landmark studies showing that both company's live, attenuated rotavirus vaccines were highly effective in preventing severe gastroenteritis in infants and didn't suffer the same safety problems that sealed the fate of the previous vaccine. Now, after periods of doubt and uncertainty, Parashar is excited. "Given the challenges and the enormous resource requirements, it is just amazing that we actually have two new products," he says.
Parashar's enthusiasm is tempered by one thing—these vaccines are yet to be tested in efficacy trials in Africa and Asia, and scientists don't know if they will be as effective at preventing severe disease in these populations as the already completed Phase III trials indicated in infants from the US, Europe, and Latin America (Science 312, 2006, 851). "That's the biggest scientific question that remains," says Parashar. The vast majority—as many as 82%—of rotavirus-related deaths occur in developing countries, but the immune responses induced by orally-administered vaccines have historically been hampered in these populations. Trials in developing countries demonstrated the need for additional doses of oral polio vaccine to stimulate equivalent immunity, and cholera vaccines and earlier versions of rotavirus vaccines performed less favorably in these settings. Before rotavirus vaccination programs can be implemented around the world, the vaccines must pass this important test.
GSK has already started two trials in Malawi and South Africa and Merck plans to initiate trials soon at sites in Africa and Asia, all of which are being conducted in cooperation with the Seattle-based public health organization Program for Appropriate Technology in Health (PATH). And although data from these studies isn't expected until 2009, organizations like the Global Alliance for Vaccines and Immunizations (GAVI; which provided PATH with a US$30 million grant), the World Health Organization (WHO), and the CDC are already actively engaged in accelerating the development and introduction of rotavirus vaccines. If implemented widely the new generation vaccines could help prevent a disease that is responsible for 5% of all childhood deaths.
Rotavirus was first identified by Australian scientists in 1973 after being isolated from intestinal samples of children suffering from diarrhea. Although named for its wheel-like appearance when viewed by electron microscopy, rotavirus' layered structure evokes an onion. The non-enveloped virus is composed of several layers of protein that facilitate cell binding and entry but also shield the central core where the 11 segments of double-stranded RNA and the associated replicative enzymes are located (Figure 2). The middle shell is composed of the most abundant viral protein, VP6, which is common to all rotaviruses. The two proteins that comprise the outermost shell—VP7, the glycoprotein or G-protein, and VP4, the protease-cleaved or P-protein—determine the serotype of the virus and are the main targets of neutralizing antibodies, making them critical in vaccine development.
Soon after the virus was discovered diagnostic assays were developed that could detect the VP6 protein and researchers began to come to grips with the burden of rotavirus disease. They quickly found that "rotavirus was the most common cause of diarrhea in children everywhere in the world," says Parashar.
So far researchers have confirmed the identity of 14 G serotypes, 14 P serotypes, and 24 P genotypes, which have not yet been assigned to a serotype and are commonly denoted within brackets (Expert. Rev. Vaccines 4, 521, 2005). Viral strains with nearly all possible combinations of these G and P serotypes have been reported in humans but, luckily for vaccine developers, there are only four combinations that predominate globally. The PG1, PG3, PG4 and PG2 account for over 80% of rotavirus-related disease, with the G1 serotype being implicated in nearly half of all rotavirus infections. The fifth most common serotype is the G9, which has been increasing in prevalence over the last decade. But Penny Heaton, head of the clinical rotavirus program at Merck, points out that the most prevalent rotavirus strain can vary widely, with several unusual strains occurring in developing countries.
The virus targets the villi of the duodenal epithelium and directly infects the cells that form the lining between the inner cavity and tissues of the intestine. Rotavirus also encodes for a peptide (nonstructural protein 4) that opens chloride channels on the surface of uninfected cells, allowing it to further wreak havoc in the gut. These two mechanisms of action trigger a range of symptoms, from mild intestinal discomfort to prolonged episodes of diarrhea and vomiting, that together account for the often rapid and severe dehydration that can result from rotavirus infection.
Oral fluid replacement is the easiest way to reverse these effects but, if necessary, fluids and electrolytes can be administered intravenously. Severe cases, however, often require hospitalization and providing treatment to the 600,000 children in the US that seek medical care for rotavirus infection costs an estimated $1 billion a year. Only between 20 and 60 children die annually from rotavirus-induced dehydration in the US. In developing countries where clean water is often a rare commodity and prompt access to healthcare services is limited, failure to re-hydrate rotavirus-infected infants and children results in a huge death toll, and around 1 in 200 children infected with rotavirus will die.
Even though rotavirus kills far more children in developing countries, the incidence is similar throughout the world. Alan Shaw, a researcher at Vaxinnate who worked previously on the development of Merck's rotavirus vaccine, calls rotavirus-induced gastroenteritis a "democratic disease" because it infects children regardless of socioeconomic status, water quality, or geographic location. Almost all infants have been infected at least once by the time they reach five years of age, making a vaccine the only hope for controlling the virus (Emerg. Infect. Dis. 9, 565, 2003).
Clues from nature
Researchers originally set out to design vaccines that could prevent the establishment of rotavirus infection, but soon changed course when studies of natural infection showed that children who are repeatedly infected with rotavirus develop a level of natural immunity that does not fully prevent subsequent infection but does reduce the risk of severe disease. Each infection grants additional protection and after two episodes it becomes unlikely that an infant will experience severe gastroenteritis. "Efforts were then focused on developing a vaccine to mimic this effect," says Parashar.
The initial approaches to rotavirus vaccines followed the classic example of Edward Jenner's smallpox strategy. Nearly all vertebrates are infected by rotaviruses and the species barrier is substantial enough that animal viruses are safe for testing in humans. This approach, using simian and bovine rotaviruses, seemed immunogenic in initial testing but protection against rotavirus was inconsistent in larger trials and the vaccine was much less effective when tested in developing countries.
The next generation of rotavirus vaccines took advantage of rotavirus' segmented genome, which allows it to reassort during coinfection. These reassortant viruses can be selected to carry the VP4 and VP7 surface proteins of human rotavirus in a simian or bovine background, allowing the reassortant to stimulate antibody production without causing disease.
Two of these reassortant vaccine candidates based on the bovine rotavirus were developed, one at the US National Institutes of Health (NIH) and the other at Children's Hospital of Philadelphia (CHOP). A third vaccine candidate, based on a rhesus macaque rotavirus, was also developed at the NIH. This simian-based rotavirus strain, known as RRV-TV, was a tetravalent strain carrying human VP7 from three different serotypes (G1, G2, and G4) and the G3 of the parent rhesus rotavirus. This live, oral vaccine candidate looked promising in early studies and was later licensed to Wyeth for large-scale safety and efficacy studies. The vaccine, called Rotashield, eventually won approval from the US Food and Drug Administration (FDA) based on studies showing that three doses were highly efficacious in preventing severe cases of diarrhea and the resultant hospitalizations caused by rotavirus infection. Shortly after licensure in 1998 the Advisory Committee on Immunization Practices at the CDC recommended vaccination with Rotashield for all infants in the US, which secured the vaccine as part of the routine immunization schedule. At this time the product had not been tested and was not available in developing countries.
Then just nine months later physicians in the US were advised by the CDC to immediately suspend use of the vaccine after the adverse events reporting system turned up an unexpected number of cases of intussusception in infants that had received Rotashield. Intussusception is a potentially fatal bowel obstruction that happens when part of the small intestine folds over itself like a collapsing telescope. It occurs naturally in 1 of every 2000 infants and requires surgical treatment in approximately 10% of cases. If left untreated it can be fatal. There were enough hints of this rare but serious side effect in pre-licensure studies to warrant a warning in the package materials that accompanied the vaccine.
Closer analysis of vaccine recipients showed an association between receipt of the vaccine and development of intussusception, with most cases occurring within two weeks after the first vaccination. A case-control study by the CDC estimated that the intussusception risk for vaccinated infants was between 1 in 4500 and 1 in 9500 (Vaccine24, 3772, 2006). "That level of risk was not considered acceptable in the US," says Parashar. Wyeth soon withdrew Rotashield from the market and stopped manufacturing the vaccine.
Risk versus benefit
This ignited debate among scientists and bioethicists on the risk/benefit calculations for vaccines. "We don't really have a risk/benefit notion of safety in the US," says Paul Offit of CHOP. But as bioethicist Charles Weijer of Dalhousie University points out it is "imperialistic to transfer this standard of care to a country in which 1 in 200 children die of rotavirus infection," (BMJ 321, 525, 2000).
Weijer calculated that even if 25% of the vaccine-induced intussusception cases proved fatal in developing countries, what he calls the worst-case scenario, it would cause 2000-3000 deaths per year, far fewer than the nearly 600,000 deaths caused worldwide each year by rotavirus-induced severe gastroenteritis. Many advocated that a vaccine that could save so many lives should still be introduced, even if it was possible that the vaccine itself would cause some deaths.
According to Parashar, all of the ethicists and most of the scientists supported testing Rotashield further in developing countries, but many representatives from these nations thought it would be politically difficult for them to promote a vaccine that was seen as unfit for infants in the US. "The risk/benefit analysis is useful and scientists understand it, but it would be difficult to explain to a layperson," adds Parashar.
Many researchers, including Parashar, now say that the biggest mistake with Rotashield—and one that resonates with AIDS vaccine researchers today—is that clinical trials in developing countries were not conducted in parallel with those in the US and Europe. "One of the challenges with this vaccine was that it hadn't already been tested in Africa and Asia," he says. Not knowing if the vaccine was even efficacious in these settings made it difficult for decision makers in developing countries to overlook the possible adverse effects.
Any discussions about testing Rotashield in developing countries soon became moot anyway because Wyeth ceased all production. The precise mechanism of Rotashield-related intussusception is still unknown, but many scientists credit it to the rapid replication of the rhesus rotavirus strain in the intestine. The peak replication for Rotashield corresponded with the occurrence of intussusception cases, says Heaton.
Further research led Lone Simonsen at the NIH to conclude that age was also a contributing factor. Infants that received the first immunization when they were older than three months, when natural cases of intussusception are more likely to occur, accounted for more than 80% of the intussusception cases reported with Rotashield. She contends that if the vaccine were only given to younger infants the relative risk of intussusception would have been greatly reduced and perhaps Rotashield would be available today (J. Infect. Dis. 192, S36, 2005). But others, including the WHO, don't support the notion that age was a dominant factor because there were very few children in these studies younger than two months with which to compare intussusception rates. In a letter to the editor Simonsen warns that regulatory authorities should discourage "catch-up" immunizations—those given to infants that haven't received their first dose at the prescribed time—with the newer vaccines because they too may cause intussusception if given to older children (N. Engl. J. Med. 354, 1748, 2006).
Small risk, huge trials
At the time the world received news about Rotashield, Merck was just preparing to take their lead rotavirus vaccine candidate, based on the bovine reassortant virus developed by Offit and colleagues at CHOP, into large-scale efficacy trials. Suddenly their plans changed dramatically. The Phase III trials needed to include 60,000-100,000 infants to successfully rule out the possibility of 1 in 10,000 vaccinees being at risk of intussusception. Both financially and organizationally, this would be a huge undertaking. However the company chose to move forward and began a placebo-controlled trial with their pentavalent rotavirus vaccine (Rotateq) in over 69,000 infants in 11 industrialized countries.
GSK was faced with a similar situation with their monovalent vaccine, known as Rotarix, based on an African green monkey/human reassortant strain initially developed at the Children's Hospital of Cincinnati, and they too pushed ahead with a trial involving 63,000 children in Finland and 11 countries in Latin America.
These trials were the largest, industry-sponsored vaccine trials ever conducted and both produced stellar results (N. Engl. J. Med. 354, 23, 2006; N. Engl. J. Med. 354, 11, 2006). Rotateq was effective at preventing 74% of any rotavirus-related gastroenteritis and 98% of severe cases, and also reduced the number of hospital visits for gastroenteritis by 86%. Immunization with Rotarix prevented 85% of severe gastroenteritis cases and associated hospitalizations and was 100% effective at reducing the most severe cases of the disease. Just as importantly, neither live-attenuated vaccine was associated with an increased risk of intussusception. "It was likely a Rotashield-specific issue," says Mark Feinberg, vice president of policy, public health, and medical affairs at Merck.
A few months after the final data were released, Merck received approval to license and market Rotateq in the US and GSK received licensure for Rotarix from the European Commission. Rotarix was already licensed in Mexico and has since also received licenses in Brazil, Philippines, and Singapore. Most recently the Advisory Committee on Immunization Practices at the CDC recommended Merck's rotavirus vaccine for all infants in the US.
These vaccines were developed without a good animal model and even after large studies proved their efficacy, researchers have yet to identify the precise correlates of protection. This gives hope to AIDS vaccine researchers who are working under similar constraints. But Offit quickly explains that "rotavirus vaccines were much easier to make," yet it still took a quarter of a century of research and development to get two safe and effective products.
Rolling out vaccines
Before the WHO will recommend rotavirus vaccination for infants in developing countries who are at the greatest risk of contracting life-threatening gastroenteritis, the vaccines must be tested in these populations. Despite the experience with Rotashield neither manufacturer chose to run efficacy trials with the second-generation vaccines concurrently in both developed and developing countries. According to Feinberg, Merck decided that the 70,000-infant study would be run only in countries where they were confident all possible cases of intussusception could be detected and treated quickly. "Now that we know the vaccine is highly efficacious and well tolerated, we want to move forward as quickly as possible in resource-poor countries," he adds.
This is happening with the help of PATH, thanks to a large grant from GAVI, which is conducting efficacy trials in partnership with both Merck and GSK in several countries in Africa and Asia. PATH's goal is to reduce the delay between when vaccines are licensed and when they become available in developing countries. The lag time for implementing hepatitis B virus vaccine programs in some countries was around 10-15 years, but they are hopeful that for rotavirus vaccines they can reduce it to about five years.
The first step is communicating with decision makers in the 72 poorest countries of the world and explaining general information about rotavirus to prepare them for eventual introduction. "If we go to countries right now and say we want to talk about rotavirus, they say what's that?" says John Wecker, who works on the rotavirus program at PATH. These countries know they have a diarrheal disease but are unaware that rotavirus is the cause. "We want to provide a solid evidence base for developing country governments, and we have a long way to go," adds Wecker.
PATH also faces many other challenges. Wecker acknowledges that when governments are aware of rotavirus, there is some difficulty getting beyond the Rotashield data and explaining to governments that these are new vaccines. "One of the biggest things that rotavirus has taught us is that safety, or perceived safety, is paramount," says Shaw.
In the future PATH will also have to explain the intricacies that differentiate Rotateq from Rotashield so that representatives from developing countries can choose which vaccine to include in their immunization programs. Wecker says this decision will be based on the serotype coverage afforded by the vaccines as well as the dosing schedule. Rotateq includes more serotypes of rotavirus and should offer a greater breadth of protection, but requires three doses. Rotarix requires only two doses but must be dissolved in a buffer solution before it can be administered.
Researchers are torn over which vaccine may work better in developing countries. One reason used to explain the historically poorer response to live-attenuated vaccines in these settings is that infants are exposed to many more intestinal bacteria and viruses, making the gut a very busy place, says Offit. And in this crowded environment where many things are competing for the immune system's attention Offit isn't sure if the vaccine strain in Rotarix, which replicates much better than the highly-attenuated strain in Rotateq, will offer an advantage.
In the end the decision on which vaccine is adopted may come down to price. PATH is now holding consultations with the manufacturers on pricing and helping them to forecast the demand for the vaccines in developing countries (see Cloudy with a chance of prevention: Demand forecasts and assessments, IAVI Report 10, 3, 2006). In the US, Merck's vaccine costs $180 for the three-dose course, making it one of the highest priced childhood immunizations. "The vaccine is priced commensurate with its public health value," says Feinberg, who also indicates that Merck is committed to making the vaccine available in developing countries at an affordable price.
Wecker is confident that financial subsidies offered through GAVI will also help poor countries. GAVI is also considering using advance market commitments (AMCs) that guarantee governments will buy a certain quantity of vaccine from a manufacturer at a negotiated price, but Wecker does not see this as a good model for rotavirus vaccines (see If you build it, they will pay, IAVI Report 9, 3, 2005). "There's no reason, from our perspective, that GAVI shouldn't move forward today, and not think about rotavirus vaccines as something that is coming in the future," he argues. "They're here today."