Vaccine development used to be straightforward. Now, the challenges are many and the successes are few. What will it take to overcome the obstacles presented by both immunology and society?
For 160 years, vaccine after vaccine succeeded at safely and effectively preventing its targeted illness using a set of standard strategies. Scientists knew they simply had to weaken or kill a virus or toxin and it was “mission accomplished.” But one fatal mistake in the 1960s rocked the field’s foundation, calling into question the future of vaccinology.
On paper, the vaccine had looked perfect. Its concept was in line with the strategy that had brought well over a dozen other vaccines to the realm of medical miracles. As the deadliest infectious diseases approached global eradication, vaccine scientists turned their sights toward respiratory syncytial virus (RSV), another threat to human health, first recognized in the 1950s. RSV continues to hospitalize millions of infants each year and causes up to 200,000 childhood deaths annually around the world. Following the process of their predecessors, RSV vaccinologists designed a vaccine containing the killed virus and formalin, a water-based solution of formaldehyde. Initial research suggested the vaccine stimulated strong immunization against RSV, and clinical studies in human infants began in Washington, D.C., in 1966.
The vaccine failed with tragic consequences, leaving vaccinologists shocked, confused and saddened. Among the study’s control children, who were vaccinated against parainfluenza, 5 percent of those who naturally developed an RSV infection ended up in the hospital, all of whom survived. By comparison, 80 percent of the RSV-vaccinated infants who developed the illness were hospitalized, and two died. Instead of receiving protection from the vaccine, the immunized children had developed a more serious form of the disease.
In the aftermath of the RSV vaccine of 1966, immunology specialists had to confront a new world of challenges in their field. Public sentiment and changing ideas about medical ethics reshaped the landscape of research on human subjects, money became a serious player in the decision-making process about vaccine research and development, and perhaps most importantly, vaccines for the “easy” diseases were already developed. In short, the RSV vaccine disaster was a dramatic wake-up call signaling a new era for vaccinology.
One Size Does Not Fit All
After nearly 1,000 years of crude inoculation strategies against smallpox, the first formal vaccination from the cowpox virus was developed by Edward Jenner in 1796. His technique gave birth to the concept of stimulating the body’s immune response with a weakened or altered virus so that, when threatened by the actual infectious disease, the body would already have the cells in place to recognize the invader and mount a rapid, effective response. This vaccination and similar vaccines for conditions such as yellow fever, influenza and polio have since saved the lives of hundreds of millions of people throughout the world.
Yet in a way, Edward Jenner’s smallpox vaccine targeted a type of disease that scientists now consider low-hanging fruit.
That’s because modern-day targets are harder to locate and refine, requiring a much deeper understanding of the organisms involved. In some cases, multiple pathogens can be responsible for a single disease. In others, the virus mutates quickly, creating a moving target for a vaccination.
Researchers now must also frequently consider using adjuvants, additives that enable a vaccine to hit its antibody and cellular targets more effectively or stimulate the immune response in a specific way. These adjuvants bridge the gap between what should work and what the body actually requires for the expected immunologic response.
The advancement of the field of vaccinology from the days of single-organism, clear-target vaccines to the challenges of today have required a host of new technologies. Microbiology and pathology are at least as important to vaccine creation as an understanding of immunology, and the many unknowns about the immune system make progress slow and difficult.
According to John Clements, PhD, professor and chair of Microbiology and Immunology at Tulane University School of Medicine, the pioneer vaccinologists were fortunate in one respect. “Early vaccines for people were against diseases like diphtheria and tetanus, for which an extracellular toxin is the primary manifestation. If you could neutralize the toxin, then you could prevent the disease,” he says. “Whole-killed or live attenuated organisms were also early targets, as with typhoid fever and polio. Most of these efforts were directed at making antibodies against the bacterium or virus.”
Now, anticipating and controlling the antibody response is only half the battle, Dr. Clements says. Scientists need to learn how to manage the cellular immune response, understanding the immune system and the exact mechanisms by which organisms cause disease. Much of Dr. Clements’ work involves adjuvant development to improve techniques for existing or promising vaccines for diseases such as tuberculosis, rotavirus, polio and diarrheal illness caused by Escherichia coli. And once a safe and effective vaccine is created, he says, the question of delivery comes into play.
Not every vaccine works best as a shot in the arm, and most vaccines require a “cold chain,” or refrigeration from creation to injection. Nontraditional delivery routes — such as intradermal, oral, sublingual and transcutaneous administration — thus offer promise for improving outcomes in resource-poor areas where people are still suffering from vaccine-preventable diseases. Dr. Clements and his research team, funded by the National Institutes of Health and the Bill & Melinda Gates Foundation, investigate ways to reduce the costs of existing vaccines, augment their function, increase their shelf life or improve their accessibility. By developing alternatives for delivery method and storage, they could reduce or eliminate the need for a cold chain or for administration by trained health care workers.
“Imagine a transcutaneous vaccine on an adhesive bandage that would allow you to skip the needle and syringe while inducing a mucosal immune response,” Dr. Clements says. “Theoretically it could be a huge advantage, since most of the pathogens we encounter first infect mucosal surfaces. If we were able to immunize through the skin and induce a response at the level of the mucosal surface, then that could stop an infection before it begins.”
Effective transcutaneous immunization may be closer than many vaccine scientists think.
Lending an Ear
Lauren Bakaletz, PhD, director of the Center for Microbial Pathogenesis in The Research Institute at Nationwide Children’s Hospital, thinks transcutaneous delivery of a vaccine could be just the ticket to preventing — and even curing — chronic ear infections in children around the world.
Her own approach to vaccination for otitis media, or middle-ear inflammation, is a novel one. It hinges on finding the right target and helping the body mount an effective immune response to particular bacterium depending on its location in the body. The bacteria known to be involved in ear infections naturally inhabit a specific region within the respiratory tract. But when an upper respiratory tract viral infection disrupts the homeostasis of the bacterium’s normal environment, it travels and begins to cause trouble.
“We don’t want to get rid of them all, because they do have some beneficial qualities,” Dr. Bakaletz explains. “So we are aiming at a way to titrate these vaccines to keep the number of bacteria from getting to the disease-causing level, because that’s when they go where they don’t belong — up into the ears, into the sinuses or down into the lungs.”
A transcutaneous vaccine that could target an immune response just where the bacteria are causing inflammation and infection could offer a solution to an undesirable system-wide attack on our natural community of bacteria. The outer layer of the skin in all mammals is arranged in a staggered pattern, like brick and mortar, forming an impermeable barrier that complicates transcutaneous vaccine delivery. But the skin behind the ear, called the post-auricular skin, is the only place in mammals, including humans, where the cells of the outer layer of skin are stacked in linear layers that are more amenable to vaccination.
“If you put a vaccine on top of that skin, cells can send dendrites up to grab that vaccine antigen and take them to the regional lymph nodes for processing,” Dr. Bakaletz explains. “This is an ideal site to immunize for a disease that’s right there — in the middle ear.”
Her research, funded by the National Institutes of Health and the National Institute on Deafness and Other Communication Disorders (NIDCD), focuses on otitis media for both a skin patch and an injectable vaccine. Dr. Bakaletz and her team hope the two strategies can yield a way to thwart and treat chronic ear infections in diverse populations.
“We’re trying to prevent ear infections from ever occurring. In this scenario, the bacteria would never get into your ear and you’d never develop chronic or recurrent disease,” Dr. Bakaletz predicts. “But if you could develop a vaccine that could cure someone with existing otitis media and also confer some sort of preventive benefits so that they don’t get otitis media again, then that would be ideal.”
Estimates put the prevalence of chronic secretory otitis media between 65 and 330 million children worldwide. These children have long-term infections causing pus to drain from their ears through perforated ear drums, which can impact their ability to hear and, in turn, limit their ability to learn language. Even curative treatments are difficult to distribute in developing countries, where the problem is most widespread. But in most cases, antibiotics are not useful for treating recurrent ear infections because of biofilms, sticky scaffolds formed by groups of bacteria that prevent the drugs from reaching the target organisms.
In the process of examining this barrier to effective antibiotic delivery, Dr. Bakaletz and her colleagues discovered that the nonmotile bacterium Haemophilus influenzae— one of the bacteria commonly involved in chronic ear and respiratory tract infections — was, in fact, motile. It twitched its way into groups that grew into the sticky biofilm scaffolds. “That discovery changed the way the whole world thought about this microorganism,” says Dr. Bakaletz of that research, published in 2005 in the journal Infection and Immunity. “To see this big protein sticking out of the bacterium that allowed it to latch onto surfaces and other bacteria in the area and then learn how important that protein was to the production of biofilms — that screamed vaccine candidate to me.”
Theoretically, if she could prevent the bacterium from sticking to a cell or from latching onto other bacteria to form the biofilm, it could be managed by drugs or the body’s immune system. The target was clear. By aiming to disrupt the function of that protruding protein in the bacteria, Dr. Bakaletz and her team showed that the bacteria were unable to effectively build the biofilms. Even better, inhibiting the protein caused the collapse of bacterial biofilms, and the bacteria then became susceptible to antibodies and antibiotics. For the first time in history, a therapeutic vaccine for this microbe was in reach.
“The fact that Dr. Bakaletz’s approach is both therapeutic and prophylactic is significant,” says Dr. Clements of her research. He learned of Dr. Bakaletz’s vaccine candidate and suggested the behind-the-ear idea that she is now developing with funding from the NIH, the NIDCD and the National Center for Advancing Translational Sciences.
“He said, ‘Why don’t you take your vaccine and rub it on the ear and see what happens?’” she recollects. “Sure enough, it worked beautifully, so we decided to put it on an adhesive bandage and place it on the skin just behind the ear in our animal models, and it worked. Again and again.”
The current version of her team’s vaccine relies on Dr. Clements’ adjuvant to improve the mucosal and systemic response to the vaccine. “Now, the onus is on us to prove how it works,” Dr. Bakaletz says of the work ahead of her team. “People are skeptics. We are skeptics. It’s one thing to see the end result but another to figure out the mechanisms at play and whether this approach could work for other diseases, too.”
The patch application of the vaccine is now in pre-clinical studies. If Dr. Bakaletz and her team succeed in bringing the vaccine to the clinical realm any time soon, it will be one of the few contemporary examples of such a feat being accomplished during the career of a single scientist. But as her collaboration with experts in adjuvant and biofilmresearch demonstrates, no researcher works in isolation when it comes to building the modern-day vaccine.
It Takes a Village…And Then Some
If any vaccine saga affirms that adage, it is perhaps that of RSV — the story with a rocky beginning and an uncertain end.
Four decades after its tragic start, Fernando Polack, MD, the Cesar Milstein Professor of Pediatrics specializing in pediatric infectious disease research at Vanderbilt University School of Medicine, and his team set out to solve the mystery of why an effective vaccine for the deadly respiratory condition is still out of reach. In 2008, Dr. Polack and his team demonstrated that the 1966 RSV vaccine failure likely resulted from not properly priming the immune system. And much like work with the otitis media vaccine, a new adjuvant may play a critical role in overcoming the RSV vaccine’s backfire.
The study, published in Nature Medicine, investigated the impact of a very similar RSV vaccine in mice with and without the addition of an adjuvant. The new adjuvant was targeted at boosting the body’s natural affinity maturation — a process by which repeated exposure to the same antigen stimulates the body’s white blood cells to respond with increased numbers of antibodies. Dr. Polack’s research showed that by activating a key pathway in the immune system’s response to the virus, the body may develop the proper protective antibodies against RSV. When a variation of the killed vaccine was administered with the new adjuvant, the mice did not suffer from the heightened disease symptoms caused by the 1966 human vaccine.
Unfortunately, the solution to the decades-long mystery may not be as simple as adding a single adjuvant that enables a better immune response. Scientists and regulators are reluctant to try a killed virus again. Instead, many researchers believe that live attenuated viruses and purified viral protein vaccines may be the best approach.
About 30 variations of a new RSV vaccine are currently in development, and although some show considerable promise, many attempts at a vaccine have already been proven ineffective.
“There’s not going to be a one-size-fits-all approach,” Dr. Clements suggests. Because of this likelihood, scientists are taking a step back and surveying the landscape of RSV vaccine creation with a systematic approach.
“There are three technical problems that are central to making a live attenuated RSV vaccine work,” says Mark Peeples, PhD, principal investigator in the Center for Vaccines and Immunity at Nationwide Children’s. “The first one is the virus has developed a very potent mechanism for preventing cells from producing interferon, the main actor in the initial innate immune response, and that likely in turn dampens the adaptive immune response, antibodies and T cells.”
The second problem, says Dr. Peeples, is related to what his own team studies. The monkey cell line that is used to grow the experimental live attenuated RSV vaccines actually weakens the virus, limiting the production of a live RSV vaccine and making it expensive to manufacture. His team has decided to work around this barrier.
“When we grow the RSV virus in monkey cells, its attachment protein, called G glycoprotein, is clipped and no longer functions, so that the virus cannot attach to its receptor on human airway cells,” Dr. Peeples says of the discovery he published in the Journal of Virology in 2009. “That means that these cell lines, which the World Health Organization approved and which most RSV vaccine scientists use, don’t work well for RSV.” Rather than studying the virus in those established cell lines, Dr. Peeples’ team isolates cells directly from the airways of organ donors, because they have discovered that RSV uses a different receptor in these cells and, most likely, in living people.
According to Dr. Peeples, the third problem is finding the best method for attenuating RSV, ideally by methodically searching for the strategy that produces viruses that are progressively weaker, then selecting the most effective ones for trial vaccines. Once these three problems are solved, Dr. Peeples believes a successful RSV vaccine will follow.
To that end, he and collaborators at The Ohio State University and the University of South Florida are using NIH funding to attack different angles of the RSV vaccine mystery. Dr. Peeples believes that their investigations and those performed by RSV vaccinologists around the world will solve the enigma during his lifetime. But the collective efforts are crucial, he says.
“The days of Jonas Salk, of inventing a vaccine by yourself, are probably behind us,” Dr. Peeples says. “RSV, HIV, hepatitis C — these are viruses that have much more complex problems than the diseases that succumbed to standard vaccine techniques for many decades. They must be attacked in a different way.”
But Dr. Peeples and his colleagues are up for the challenge. “The good news is that we’ve learned a lot about these organisms and developed a lot of tools that we didn’t have back in those days,” he says.
“I” is for Immunome
The widespread focus on various strategies and the tools developed over the last 50 years may help vaccinologists address a range of challenges confronting the field. For instance, what is the best age for administering a vaccine? How long will immunity last? How much risk is acceptable?
James Crowe, Jr., MD, an immunologist, board-certified pediatric infectious disease specialist and director of the Vanderbilt Vaccine Center, wants to find answers to those and other questions. He is a vocal advocate for a Human Vaccines Project, modeled after the Human Genome Project, with the goal of pushing vaccine science forward in part by mapping and analyzing the human “immunome.” To Dr. Crowe, this effort could hold the key to propelling vaccinology toward an effective vaccine for RSV, HIV and other complex diseases.
“We need to step back to ask more fundamental questions about what components the immune system is comprised of — we need a ‘parts’ list,” says Dr. Crowe, who believes that researchers may be able to harness the appropriate information to create the next generation of vaccines. “Tools are emerging that will allow us to understand the underlying mechanisms of the immune system, which could be applied to all manner of disease targets.”
The human immunome has a number of gene segments that are put together in combinations to make antibodies or T cell receptors, Dr. Crowe explains. The number of potential combinations of these segments is estimated to be 1013, boiling the science of the immune system down to a problem of data.
“It’s an ambitious 10-year goal,” Dr. Crowe says. “Once we have a list of all the antibodies and T cell receptors that the human population can make, we’d also have to understand how they relate. We need to connect the scientific community with the big data community and use those types of tools for large-scale storage and data analysis that are currently being used for commercial purposes.”
According to Dr. Crowe, when these substantial accomplishments are achieved, scientists can move toward rationally developing vaccines that are tailored to fit typical human immune responses. In the case of RSV, Dr. Crowe and his collaborators used an approach called reverse vaccinology to identify the antibody they wanted the immune system produce and then worked backward to figure out where the antibody would fit onto the virus. In collaboration with colleagues who designed a small protein on the computer that mimicked the antibody-binding site of RSV, Dr. Crowe’s team was then able to induce immunity and stimulate antibody production in animal models that were nearly identical to the one they had expected.
“We manipulated the immune system of animals to make the antibody we had created on the computer,” says Dr. Crowe, who believes this application of vaccine study is just scraping the surface of what will be possible with computer design in the future.
Where Science and Society Meet
In the modern-day development and delivery of vaccines, creating an effective vaccine and knowing how best to administer it are only part of the picture. Standards for medical ethics and safety, as well as the social, political, ethical and financial climates surrounding vaccine delivery, present challenges for every scientist bringing a new vaccine to clinical trials.
“There is a tremendous safety barrier — society has an exceptionally low tolerance for adverse events,” Dr. Crowe says. This is even the case for vaccines targeting otherwise pervasive, deadly diseases.
“Any vaccine has to pass a very high bar,” Dr. Peeples agrees. Vaccine scientists understand the need to not only minimize actual risk, but also to assuage societal concerns, which can make or break a vaccine’s success once brought to market.
“There is a tremendous amount of responsibility associated with developing something that’s going to be injected into humans, and I think that responsibility is even greater when the population you’re targeting is newborn babies,” Dr. Bakaletz says. “You have to do the work very diligently.”
But even a high safety profile and demonstrations of success may not be enough to influence popular opinion in some cases, these researchers suggest.
“People get resistant or lackadaisical about vaccinating and don’t realize that effective, robust immunization programs are what keep diseases away,” Dr. Bakaletz says. “There has to be constant vigilance, people have to become more aware. And now that many vaccine-preventable diseases are recurring, it is sparking a new societal conversation on the importance of vaccinations.”
Although broad social support may begin to swell in the aftermath of vaccine-preventable disease outbreaks, these researchers say that the path to vaccine distribution depends most heavily on one thing: money. Funding determines which research advances, priorities at pharmaceutical companies dictate which vaccines will be advantageous to pursue, and countries must decide which vaccines make the best use of limited health care dollars.
“You’re typically looking at over $1 billion in research and development to get a reproducible, affordable vaccine to market,” says Dr. Crowe, whose work on the reverse-engineered RSV vaccine is currently in preclinical studies. “The vaccine business is not a high-margin industry. But if you’ve been a medical provider and held a baby who died in your arms, it changes your view on life. You want to do something about it. Money and technical obstacles pale in comparison to that.”
His dedication to the end goal of vaccinology — saving human lives — is not unique. The passion and determination to protect human life, not make money, is a common motivator among vaccine scientists.
“We’re all on the same page: We want these vaccines developed and we don’t want cost to be the reason they’re not,” Dr. Clements says. His own work with adjuvants is licensed by PATH and Tulane, who will license them to anyone, royalty free, for use in developing countries.
Understandably, vaccinologists don’t want their efforts to be in vain. “If you make a good vaccine, it should be used,” Dr. Peeples says. “But if nobody’s going to produce it because of money, what’s the point?”
“Political, financial, societal and cultural issues have always presented challenges to vaccine development and acceptance programs,” Dr. Bakaletz concedes. “But there is so much proof that they are the most cost-effective way to manage infectious diseases, they must have a place in the future of medicine. Access to a vaccine shouldn’t be defined by money or politics.” Her work with otitis media and biofilms may also hold clues impacting other diseases of the airway, including chronic obstructive pulmonary disease and even cystic fibrosis.
Once scientists understand the biological mechanisms at work in the human immune system and their target diseases, they can apply the latest technology toward interrupting or preventing the infection process with their vaccines. At that point, some believe the vaccine floodgates will open.
“When we overcome these scientific challenges, the field of opportunity for effective vaccines won’t be limited to only infectious diseases,” Dr. Crowe says. “Think of cancer vaccines, therapeutic vaccines for chronic illness — we should take a longer-term view of what’s going to come out of these basic science projects.”
Despite all the barriers, vaccinologists aren’t discouraged, and many teams work at the cusp of critical discoveries that can advance the science of immunization. They are tackling the difficulties presented by their target diseases from every conceivable angle, and laboratory techniques are keeping pace with advances in their understanding of disease mechanisms.
They are up for the challenge, and the future of vaccinology, though complicated, looks bright.