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, increasing protection against non-targeted infections, but also at times negative, increasing susceptibility to non-targeted infections. This depends on both the vaccine and the sex of the infant.

All live attenuated vaccines studied so far (BCG vaccine, measles vaccine, oral polio vaccine, smallpox vaccine) have been shown to reduce mortality more than can be explained by prevention of the targeted infections. In contrast, inactivated vaccines (diphtheria-tetanus-pertussis vaccine (DTP), hepatitis B vaccine, inactivated polio vaccine) may increase overall mortality despite providing protection against the target diseases.

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 side 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.

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