Inside the cell, the virus rapidly reproduces, killing the cell and distributing copies of itself to attack other cells. The immune system responds by creating antibodies or sending white blood cells to fight the virus. Against relatively weak viruses, the immune system triumphs. In other cases—a virulent case of polio, for example—the body is overwhelmed and the patient dies or suffers lasting injury. Antibiotics, designed to kill invading bacteria, are useless against viruses, which do their damage while hiding inside the body’s own cells.
One of the few good things about this process is that once you survive a viral infection, the antibodies you developed usually make you immune to future attacks of the same disease. That’s the idea behind vaccines: to inject a substance that, though harmless, is close enough in composition to the actual virus to stimulate the development of protective antibodies.
Most vaccines have been made in one of two ways. The first is to use heat or chemicals to kill samples of the virus. Though the virus is dead and thus no longer at risk of reproducing inside cells, its presence is enough to stimulate an immune response. The second method, which Hilleman used to create his mumps vaccine, involves growing and regrowing viruses in animal cells, a process known as attenuation. With each generation, the virus mutates slightly, gradually becoming less dangerous to humans, until it can be safely injected.
Vaccines made from attenuated viruses tend to be more effective than those made from dead viruses. But live viruses have the potential to cause disease, and the process of creating vaccines with them is cumbersome and inefficient. For one thing, it requires enormous supplies of fresh animal parts—often, specially bred embryonic chicken eggs. Yet that’s still how many vaccines, including those for each year’s flu shots, are made. The months required to cultivate and procure eggs bred for uniformity and disease resistance is a major reason the federal government must forecast expected flu strains months before flu season hits—and why its predictions can turn out to be wrong, as seems to have happened this past winter.
Making a vaccine this way is based solely on laborious trial and error. New production methods, in contrast, are much more efficient and precise. Scientists are increasingly using man-made cell cultures rather than chicken or other animal cells to grow flu and other viruses. These cell cultures can reproduce rapidly in the laboratory, reducing the time and expense associated with animal cells. The new method requires re-equipped factories and retrained employees, so the transition is taking time, but it’s already helping deliver vaccines more quickly and profitably than before.
Another breakthrough has been the use of recombinant DNA technology to replicate viral DNA. The genetic material, engineered without its disease-causing characteristics, nevertheless spurs the immune system to fight back. In that way vaccines can be created in a single step and administered with an air gun that implants a DNA particle under the skin. The method requires much less training than administering conventional vaccines.
If Gardasil is the poster child for a new golden age of vaccines, it isn’t the only success story. In 2006, Merck and GlaxoSmithKline introduced vaccines for rotavirus, a leading cause of life-threatening diarrhea in infants, especially in developing countries. Merck has also introduced a vaccine to prevent shingles that has been approved for use by people age 60 and older. (Merck says its vaccines sales were $4.3 billion in 2007, compared with $1.9 billion in 2006.) Another new vaccine, Wyeth’s Prevnar, protects children against some types of pneumococcal infections and is the first vaccine to exceed $2 billion in annual sales. |
