Aren’t these diseases are so rare that vaccination is no longer necessary?

One of the reasons that the incidence of vaccine preventable diseases is now so low is because of wide scale vaccination efforts.  When the majority of people in a community are immunized, there is there are fewer people who are able to spread the disease.  Therefore, people who are susceptible to disease are also less likely to contract the disease because the majority of people they come into contact with do not carry it.  This is called herd immunity.  When vaccination levels fall, there are often outbreaks of disease.  “Three countries - Great Britain, Sweden, and Japan - cut back the use of pertussis vaccine because of fear about the vaccine. The effect was dramatic and immediate. In Great Britain, a drop in pertussis vaccination in 1974 was followed by an epidemic of more than 100,000 cases of pertussis and 36 deaths by 1978. In Japan, around the same time, a drop in vaccination rates from 70% to 20%-40% led to a jump in pertussis from 393 cases and no deaths in 1974 to 13,000 cases and 41 deaths in 1979. In Sweden, the annual incidence rate of pertussis per 100,000 children 0-6 years of age increased from 700 cases in 1981 to 3,200 in 1985. It seems clear from these experiences that not only would diseases not be disappearing without vaccines, but if we were to stop vaccinating, they would come back.”

Herd immunity:   

Herd immunity is important because even if there was 100% vaccination rate, not everyone who received the vaccine would respond to it.  Most vaccines work in 85-95%  of people.  There are also a small percentage of people who can not receive immunizations because of various conditions.  Some people can not receive live vaccines such as measles or varicella because they are immunocompromised and even the weakened strands of virus could be dangerous for them.  If the remainder of the community has received vaccination, these people are protected because those around them are not going to transmit the disease.  For these individuals, their only protection is through those around them. When people opt out of vaccinations, they are weakening the heard immunity.  They are allowing an opportunity for not only their own illness, but also for possible transmission of the illness to others who may be severely affected by the disease.  People who opt out of vaccinations are ironically relying on herd immunity for their own protection while weakening it for others. 

Examples of herd immunity at work.

Consider the following graph of deaths from varicella.  In this graph, the cases are divided by age.  One of the largest declines is in the under 1 year old category.  But the varicella vaccine is only given to children older than 12 months old.  An explanation for this decline is that by vaccinating those around them, such as siblings, the infants were less likely to have contact with  the disease and thus were less likely to contract it and die from it. 

            A similar approach is used for infants and influenza vaccine.  Infants under 6 months old do not respond effectively to the influenza vaccine (flu shot), so it is not recommended for them.  It is recommended, however, for all of their household contacts.  By immunizing the household members, the infant is also protected as these are the people from whom the infant is most likely to contract the disease. 

Links:

NIP: Common Misconceptions About Vaccines