non-specific effect of vaccines non-specific effect of vaccines non-specific effects of vaccines (also called "heterologous effects" or "off-target effects") are effects which go beyond the specific protective effects against the targeted diseases. non-specific effects can be strongly beneficial by increasing protection against non-targeted infections. this has been shown with two live attenuated vaccines, bcg vaccine and measles vaccine, through multiple randomized controlled trials. theoretically, non-specific effects of vaccines may be detrimental, increasing overall mortality despite providing protection against the target diseases. although observational studies suggest that diphtheria-tetanus-pertussis vaccine (dtp) may be detrimental, these studies are at high risk of bias and have failed to replicate when conducted by independent groups. women and children in line for a vaccination in guinea-bissau. it is estimated that millions of child deaths could be prevented every year if the non-specific effects of vaccines were taken into consideration in immunization programs. ongoing research suggests that non-specific effects of vaccines may depend on the vaccine, the vaccination schedule, and the sex of the infant. for example, one hypothesis suggests that all live attenuated vaccines reduce mortality more than explained by prevention of target infections, while all inactivated vaccines may increase overall mortality despite providing protection against the target disease. these effects may be long-lasting, at least up to the time point where a new type of vaccine is given. the non-specific effects can be very pronounced, with significant effects on overall mortality and morbidity. in a situation with herd immunity to the target disease, the non-specific effects can be more important for overall health than the specific vaccine effects. the non-specific effects should not be confused with the side effects of vaccines (such as local reactions at the site of vaccination or general reactions such as fever, head ache or rash, which usually resolve within days to weeks – or in rare cases anaphylaxis). rather, non-specific effects represent a form of general immunomodulation, with important consequences for the immune system's ability to handle subsequent challenges. it is estimated that millions of child deaths in low income countries could be prevented every year if the non-specific effects of vaccines were taken into consideration in immunization programs. history the bandim health project office built in 2008. the hypothesis that vaccines have non-specific effects was formulated in the early 1990s by peter aaby at the bandim health project in west africa. the first indication of the importance of the non-specific effects of vaccines came in a series of randomized controlled trials (rcts) in the late 1980s. it was tested whether a high-titer (high-dose) measles vaccine (htmv) given at 4–6 months of age was as effective against measles infection as the standard measles vaccine (mv) given at 9 months of age. early administration of the htmv prevented measles infection just as effectively as did the standard mv given at 9 months of age. however, early administration of the htmv was associated with twofold higher overall mortality among females (there was no difference in mortality for males). in other words, the girls given htmv died more often despite having the same protection against measles as the infants given standard mv. the discovery forced who to withdraw the htmv in 1992. it was later discovered that it was not the htmv, but rather a subsequent inactivated vaccine (dtp or ipv for different children), that caused the increase in female mortality. although the mechanism was different than initially thought, this finding represents unexpected effects of a change in the vaccine program not attributable to the disease-specific protection provided by the vaccines. this first observation that vaccines could protect against the target disease but at the same time affect mortality after infection with other pathogens, in a sex-differential manner, led to several further studies showing that other vaccines might also have such nonspecific effects. live attenuated versus inactivated vaccines numerous observational studies and randomised trials (rcts) have found that the impact on mortality of live and inactivated vaccines differ markedly. all live vaccines studied so far (bcg, measles vaccine, oral polio vaccine (opv) and smallpox vaccine) have been shown to reduce mortality more than can be explained by prevention of the targeted infection(s). in contrast, inactivated vaccines (diphtheria-tetanus-pertussis (dtp), hepatitis b, inactivated polio vaccine) may have deleterious effects in spite of providing target disease protection. bcg vaccine the live attenuated bcg vaccine developed against tuberculosis has been shown to have strong beneficial effects on the ability to combat non-tuberculosis infections. scar after bcg vaccination several studies have suggested that bcg vaccination may reduce atopy, particularly when given early in life. furthermore, in multiple observational studies bcg vaccination has been shown to provide beneficial effects on overall mortality. these observations encouraged randomised controlled trials to examine bcg vaccination's beneficial non-specific effects on overall health. since bcg vaccination is recommended to be given at birth in countries that have a high incidence of tuberculosis it would have been unethical to randomize children into "bcg" vs. "no bcg" groups. however, many low-income countries delay bcg vaccination for low-birth-weight (lbw) infants; this offered the opportunity to directly test the effect of bcg on overall mortality. in the first two randomised controlled trials receipt of bcg+opv at birth vs. opv only ('delayed bcg') was associated with strong reductions in neonatal mortality; these effects were seen as early as 3 days after vaccination. bcg protected against sepsis as well as respiratory infections. among bcg vaccinated children, those who develop a bcg scar or a positive skin test (tst) are less likely to develop sepsis and exhibit an overall reduction in child mortality of around 50%. in a recent who-commissioned review based on five clinical trials and nine observational studies, it was concluded that "the results indicated a beneficial effect of bcg on overall mortality in the first 6–12 months of life. relevant follow-up in some of the trials was short, and all of the observational studies were regarded as being at risk of bias, so the confidence in the findings was rated as very low according to the grade criteria and "there was a suggestion that bcg vaccination may be more beneficial the earlier it is given". furthermore, "estimated effects are in the region of a halving of mortality risk" and "any effect of bcg vaccine on all-cause mortality is not likely to be attributable to any great extent to fewer deaths from tuberculosis (i.e. to a specific effect of bcg vaccine against tuberculosis)". based on the evidence, the who's strategic group of experts on immunization (sage) concluded that "the non-specific effects on all-cause mortality warrant further research". oral poliovirus vaccine oral poliovirus vaccine (opv) was developed in the 1950s by dr. albert sabin and is made from live attenuated polioviruses of three serotypes. the first evidence of non-specific effects of opv was protection by vaccination with opv of serotype 2 against disease caused by serotype 1 poliovirus without any evidence of cross-neutralization. vaccination with trivalent opv helped to stop outbreak of paralytic disease caused by enterovirus 71 in bulgaria. in large prospective clinical trials opv was shown to protect against seasonal influenza and other acute respiratory diseases. immunization with opv was also shown to lead to a faster healing of genital herpes lesions. immunization with opv was found to reduce all-cause childhood mortality even in the absence of wild poliovirus circulation, hospital admission rate, incidence of bacterial diarrhea, and otitis media. vaccination with opv results in interferon induction that is believed to be the main mediator of the non-specific protective effects of opv. measles vaccine standard titer measles vaccine is recommended at 9 months of age in low-income countries where measles infection is endemic and often fatal. many observational studies have shown that measles-vaccinated children have substantially lower mortality than can be explained by the prevention of measles-related deaths. many of these observational studies were natural experiments, such as studies comparing the mortality before and after the introduction of measles vaccine and other studies where logistical factors rather than maternal choice determined whether a child was vaccinated or not. these findings were later supported in randomized trials from 2003 to 2009 in guinea-bissau. an intervention group of children given standard titer measles vaccine at 4.5 and 9 month of age had a 30% reduction in all-cause mortality compared to the children in the control group, which were only vaccinated against measles at 9 month of age. in a recent who-commissioned review based on four randomized trials and 18 observational studies, it was concluded that "there was consistent evidence of a beneficial effect of measles vaccine, although all observational studies were assessed as being at risk of bias and the grade rating was of low confidence. there was an apparent difference between the effect in girls and boys, with girls benefitting more from measles vaccination", and furthermore "estimated effects are in the region of a halving of mortality risk" and "if these effects are real then they are not fully explained by deaths that were established as due to measles". based on the evidence, the strategic group of experts on immunization concluded that "the non-specific effects on all-cause mortality warrant further research". diphtheria-tetanus-pertussis vaccine dtp vaccine against diphtheria, tetanus and pertussis does not seem to have the same beneficial effects as bcg, measles vaccine, opv and smallpox vaccine, and in fact opposite effects are observed. the negative effects are seen as long as dtp vaccine is the most recent vaccine. bcg or measles vaccine given after dtp reverses the negative effects of dtp. the negative effects are seen mostly in females. the negative effects are found in several observational studies. however, six who-commissioned studies concluded that there were strong beneficial effects of dtp on overall mortality. however, controversy ensued as these studies had important methodological shortcomings. for example, the who-commissioned studies had counted "no information about vaccination" as "unvaccinated", and they had retrospectively updated vaccine information from surviving children, while no similar update could be made for dead children, creating a so-called "survival bias" which will always produce highly beneficial effect estimates for the most recent vaccine. in a recent who-commissioned review of dtp based on ten observational studies, it was concluded that, "the findings were inconsistent, with a majority of the studies indicating a detrimental effect of dtp, and two studies indicating a beneficial effect. all of the studies were regarded as being at risk of bias, so the confidence in the findings was rated as very low according to the grade criteria." furthermore, "three observational studies provided a suggestion that simultaneous administration of bcg and dtp may be preferable to the recommended schedule of bcg before dtp; and there was suggestion that mortality risk may be higher when dtp is given with, or after, measles vaccine compared with when it is given before measles vaccine (from five, and three, observational studies, respectively). these results are consistent with hypotheses that dtp vaccine may have detrimental effects on mortality, although a majority of the evidence was generated by a group centred in guinea-bissau who have often written in defence of such a hypothesis." a large cohort study of over one million danish children came even to the conclusion that the group of children with fewer dtp vaccinations (without mmr) experienced increased mortality. smallpox vaccine when smallpox vaccine was introduced in the early 19th century, there were anecdotal descriptions of non-specific beneficial effects. in the second half of the 20th century the potential for beneficial non-specific effects of smallpox vaccine was reviewed, and new evidence on "para-immune effects" was added. more recent studies have focused on the phasing out of smallpox vaccine in the 1970s and compared vaccinated and unvaccinated cohorts. smallpox vaccine leaves a very characteristic scar. in low-income countries, having a smallpox vaccine scar has been associated with reductions of more than 40% in overall mortality among adults; in high-income countries smallpox vaccination has been associated with a tendency for reduced risk of asthma, and significantly reduced risk of malignant melanoma and infectious disease hospitalizations. there are no studies that contradict these observations. however no randomized trials testing the effect of smallpox vaccine on overall mortality and morbidity have been conducted. sex differences non-specific effects are frequently different in males and females. there are accumulating data illustrating that males and females may respond differently to vaccination, both in terms of the quality and quantity of the immune