Herd Immunity
The eradication of smallpox was one of the most significant achievements of modern medicine. It was possible due to an effective vaccine, coupled with global vaccination programs. Theoretically, it is possible to eradicate other diseases such as measles or polio in a similar manner; if enough of the global population could be vaccinated, then these diseases would cease to exist. We have come tantalizingly close to eradication in some cases: In 2000, the Center for Disease Control and Prevention declared that measles had been eliminated from the United States. Sixteen years later, the Pan American Health Organization announced that measles had been eradicated from the Americas. Polio is now endemic in only three countries in the world. Infectious diseases that routinely killed young children are now preventable thanks to childhood vaccination programs. Yet despite these milestones, there have been several outbreaks of vaccine-preventable diseases in recent times. How can this be?
A significant reason for this unfortunate resurgence is that many people, despite evidence to the contrary, view vaccine efficacy and safety as a matter of opinion, rather than one based on scientific fact. This has serious consequences, not just for individuals who choose to avoid vaccines, but also for public health initiatives as a whole.
To be effective, vaccination strategies for contagious diseases rely on a scientific concept known as "herd immunity." Herd immunity can be considered a protective shield that prevents unvaccinated people from coming into contact with the disease, thus stopping its spread. Herd immunity is particularly important for people who cannot be vaccinated, including infants, pregnant women, or immunocompromised individuals. The required level of immunization to attain benefit from herd immunity varies for each disease, and is calculated based on the infectious agent’s reproductive number—how many people each infected person goes on to infect, on average. For measles, which can cause about eighteen secondary cases for each infected person, the required level of immunization for herd immunity is about 95%. In other words, at least 95% of the entire population must be immune to prevent the spread of measles following an infection. Low vaccination levels are failing to provide protection through herd immunity, stripping one of the greatest tools in public health of its power.
Vaccines work by imitating an infection, thereby helping the body’s own defences to be prepared in case of an actual infection. Unfortunately, no vaccine is 100% effective, and the immunity provided by vaccines can wane over time; these facts are often cited by anti-vaccine activists in an effort to discredit the entire concept of vaccination. But even waning immunity is better than no immunity; for example, the smallpox vaccine was generally thought to be effective for seven to ten years, but a recent analysis showed that even individuals who were vaccinated up to thirty-five years ago would still have substantial resistance to a smallpox infection. It is undeniable that vaccines can still offer protection in the event of an outbreak, and herd immunity helps prevent the spread of disease.
The concept of herd immunity also applies to the annual flu vaccine. Unlike vaccines for measles or polio, the flu vaccine needs to be given every year because the influenza virus evolves rapidly. And because the influenza virus is not as infectious as measles, only half the population needs to be immune to prevent the spread of disease. Herd immunity protects us from the common circulating variations of the flu while the annual vaccine will protect us from the new versions that have "escaped" the existing immune response. Without a vaccine and herd immunity, a far greater number of people would be infected each year with the flu.
Vaccines are one of the greatest successes of public health. They have helped us conquer diseases such as smallpox and polio, helping us live longer, healthier, more productive lives. And yet, because of decreasing levels of vaccination, the threshold required to provide protection through herd immunity becomes unattainable; as a result, previously eradicated diseases are starting to reappear. A vaccine can be seen as an act of individual responsibility, but it has a tremendous collective impact. Vaccination on a large scale not only prevents disease in an individual, but also helps protect the vulnerable in a population. To convince the general public of its necessity and encourage more people to get vaccinated, the concept of herd immunity must be more widely understood.
