Serological evidence indicates that measles virus (MV) could circulate in seropositive, fully protected populations. Among individuals fully protected against disease, those prone to asymptomatic secondary immune response are the most likely to support subclinical MV transmission. The serological characteristics of protected subjects who developed secondary immune response after reexposure to measles have been described recently [Huiss et al. (1997): Clinical and Experimental Immunology 109:416-420]. On the basis of these data, a threshold of susceptibility was defined to estimate frequencies of secondary immune response competence in different populations. Among measles, late convalescent adults (n = 277) and vaccinated high school children (n = 368), 3.2-3.9% and 22.2-33.2%, respectively, were considered susceptible to secondary immune response. A second vaccination did not seem to lower this incidence. Even when estimates of symptomatic secondary immune response (e.g., secondary vaccine failure) were taken into account, susceptibility to subclinical secondary immune response was still 5-8 times higher after vaccination than after natural infection. Although viral transmission between protected individuals has never been directly demonstrated, the data describe a population in which protected but infectious persons could potentially be of epidemiological importance.

The research data from the above paper illustrates that transmission of disease via "protected individuals" (through vaccination) is something that can occur, an in fact it does occur as can be seen from instances of measles outbreak in highly or fully vaccinated populations.

Also re-vaccination does not protect a person from re-infection as the authors of the paper state, and that susceptibility to re-infection is 5-8 times higher after vaccination than after a natural infection - illustrating the difference between artificial and natural immunity.

Artificial Herd Immunity (through vaccination) is something that can never be achieved. In fact vaccination policies are resulting in the pushing of disease occurrence into other age groups in which it is much more dangerous - such as measles now being found in infants and adults.

The original concept of Herd Immunity was coined by A.W. Hedrich in the 1930s after he studied measles for a few decades. From his observations he concluded that when 68% of a population get the disease naturally, the outbreaks died out and stopped. This protection stopped when the number who contracted this disease or who were exposed to it fell below 68% of the population.

Unfortunately, the concept of Herd Immunity has been hijacked from its original intended meaning and is now applied to how many people are required to be vaccinated against a disease for outbreaks to completely stop. The two situations are not the same. The underlying assumption that vaccination provides the same level and type of immunity (on an individual and population level) as natural disease. There are numerous problems with this. Firstly, an artificial vaccine-induced antibody response is not the same as the more complex and complete, holistic response to a natural infection. Secondly, antibodies on their own do not necessitate protection from disease. People with high levels of antibody to a particular disease still get the disease, and people with no detectable antibodies can be immune to the disease. Thirdly, in artificially-induced immunity, antibodies wane over time and thus "booster" shots always have to be recommended whereas in a natural infection, the protection is usually life-long, if not long-term.