By Romina Libster*
Vaccines are some of the great successes in public health internationally. They not only prevent diseases but also protect us from them and can prevent serious disabilities. A long time ago, when they did not exist, millions of people around the world died from diseases such as the smallpox, polio, measles, whooping cough, rubella or hepatitis B, and others.
It is estimated that it prevents up to three million deaths of children per year and other three millions of lives are lost in the entire world due to conditions that can be prevented with existing vaccines.
In general, when we are vaccinated, we think that it is only to protect ourselves from different diseases because the immediate benefit of vaccination is individual protection, that is to say, to develop immunity to a certain pathogen.
In part, this is true, however, vaccination has a plus, a secondary benefit that is not so present for us. It is about an indirect effect in the population for it protects other members of the community who are not immunized, attributable to the presence and proximity of the individuals who are immune.
People who are not immune to a disease, either because they cannot receive the vaccine – due to age, allergies, immunodeficiencies- or they do not have access to them or even those ones who were vaccinated but could not acquire enough immunity, are protected in an indirect way by the individuals who did it. This conclusion is based on the fact that the ones who are vaccinated do not contract the disease therefore they will not transmit it to the susceptible ones. This effect is called herd or community immunity.
People who are not vaccinated almost exclusively depend on herd immunity to protect themselves from certain diseases. The social benefits of this indirect defense have important implications for public health. The people who are vaccinated take care for themselves and help to protect the whole community.
This concept, applied to many infectious diseases that have a vaccine available, can only be developed if a certain percentage of the population is immune. This number is different for each infectious agent and is called threshold and is related to different factors such as the characteristics of the pathogens, the population and the immunity the vaccine generates. In order to reap the effect of herd immunity, the percentage of immunized people should be above that threshold.
When the number of immune people in one population decreases below this threshold, diseases that used to be controlled could reappear. For instance, in different parts of the world, there were measles outbreaks, the most recent one in the California, USA.
One of the main concerns when one decides to receive a vaccine is the possible side effects. It is relevant to highlight is that a vaccine needs to be studied for many years and meet very stringent safety requirements to be approved. The scientists who develop them are very cautious and once they are authorized and used massively, they are still monitored. All vaccines have possible side effects; nevertheless, most of them are insignificant and temporary.
A community with a high percentage of immunized people can save lives. It is important to be protected not only when the disease is frequent or during an epidemic, it is also necessary when it is not so recurrent so that it does not have new outbreaks. Vaccination protects communities and saves lives.
*Romina Libster is a CONICET assistant researcher at the Fundación para la Investigación en Infectología Infantil INFANT. She is an associate professor at the Department of Pediatrics of the Vanderbilt University in the USA.
On February 25th of this year, the talk that Libster delivered at the TEDxRíodelaPlata event in October 2014 was posted on TED (http://www.ted.com/) and it reached over 80,000 views.
Link to the talk: http://www.ted.com/talks/romina_libster_the_power_of_herd_immunity#t-79377