The real risks associated with vaccinations

Vaccinations are the most effective tool for protection against infectious diseases. Thanks to this health intervention, some diseases, such as smallpox, have been eliminated. Others, however, are still widespread and can cause serious pathologies such as, for example. measles, which can have complications including pneumonia (1 in 20 cases), encephalitis (1 in 2,000 cases) or even death (1 in 3,000 cases). (Table 1).

Vaccinations are therefore recommended on a large scale with the aim of eradicating diseases as well as protecting the individual. A vaccine is an organic product that can consist of:

  • whole viral or bacterial agents, live and attenuated or inactivated or killed (measles-mumps-rubella-varicella, antipoliomyelite)
  • components of the micro-organism (anti-influenza) or substances synthesised by it (anti-diphtheria, anti-tetanus) or proteins obtained synthetically (anti-hepatitis B)
  • polysaccharide antigens conjugated with supporting proteins to increase their immunogenicity (anti pneumococcus, antimeningococcus, antihaemophilus)

The components of the vaccine are able to stimulate the immune system without causing the manifestations of the infection we wish to prevent. These components, called antigens, depending on the type and method of preparation, may require the addition of substances (adjuvants) that help the immune system develop better and more lasting protection. Some vaccines, like some drugs, require the addition of minimal amounts of preservatives, stabilisers or antibiotics, so as to ensure their stability over time and prevent bacterial growth. These added substances are also strictly controlled to confirm their safety in children and adults. Each vaccine, in fact, before being approved is subjected to lengthy experimentation to assess its tolerability and safety as well as the ability to induce an effective and lasting immune response, both when administered individually and in association with other vaccines. Even after authorisation to use, the surveillance of the safety of the products and their compatibility in combination with each other continues through a constant detection of adverse events.

Thanks to vaccinations, some diseases have been eliminated

All vaccines are subjected to rigorous trials and controls to ensure their safety and efficacy

The immune system and vaccinations

Over the millennia, the human immune system has evolved to defend itself against a wide variety of microbes: each newborn inherits this ability that manifests itself from the first hours of life. If this were not the case, the human species could not have survived. As a result the newborn is able to respond even to small parts of the microbes contained in the vaccines without being weakened. Furthermore, after overcoming an infectious disease, the immune system produces cells and antibodies that preserve its memory so that it can readily react to subsequent exposure. Vaccines also stimulate our body to produce antibodies as happens in the case of natural infection, but prevents the disease and its possible complications.

Scientific research has shown that our body, as early as the first months of life, responds adequately to vaccines even when they are administered in combination. Recent years have seen the creation of conjugate vaccines that are able to stimulate the immature immune system of the young child to protect it against some bacterial agents responsible for meningitis. The availability of these formulations allow for an early start to vaccinations, in order to protect the newborn when the defence given by maternal antibodies is reduced and therefore the risk of infections and complications is greater.

Risk assessment related to vaccinations

If by the definition of "safe vaccine" we mean a product that is totally free of side effects, then no vaccine is 100% safe. Exactly like no human activity is safe: a certain risk, however small, is inherent in all our activities. If, on the other hand, “safe” means a vaccine that can very rarely or exceptionally cause serious side effects which are considered acceptable precisely because that vaccine defends against a greater danger represented by the disease, then the definition is more realistic. Currently available vaccines must pass stringent safety tests before being approved for use (in the US by the FDA, in Europe by the EMEA, in Australia by the Therapeutic Goods Administration).

These tests are mandatory by law and are usually performed during the vaccine’s preparatory phases. Furthermore, the safety of vaccines is monitored once they come into use, by the reporting of adverse drug events.

Most of the events thought to be related to the administration of a vaccine are not actually caused by the vaccine itself. In fact, many of the events are random, that is to say events that are temporally correlated with vaccination. This is especially true during the first year of a child's life, when vaccination occurs so regularly that many events coincide with the time after which a vaccine has been administered.

An adverse vaccine event is an unwanted manifestation that occurs after a vaccination. In some cases they are signs of normal response of the organism (e.g. fever), or in rare cases they are linked to an individual predisposition (e.g. febrile convulsions). Sometimes however these manifestations are coincident, that is they occur in the days following administration, but they are not caused by the vaccine. When there is certainty that the vaccine has caused a particular event it is called an adverse reaction. The citizen can contact the attending physician and the vaccinator if suspicious symptoms arise after vaccination. However, adverse vaccine events must be evaluated by experienced personnel who can establish the correlation with vaccination with scientific criteria. In the event of an adverse event occurring after vaccination, the health worker collects the information necessary to investigate the nature of the event and check whether it is possible to proceed with the vaccination cycle or if it is necessary to suspend it and whether there are contraindications to the administration of other vaccines. In order to assess the risk associated with the use of vaccinations, we consider it more appropriate to consider the concept of safety as "the ability to protect against a real danger". Using this definition, the danger (disease) must be significantly greater than the means to protect against it (vaccine). In other words, the benefits of a vaccine must clearly and definitively outweigh its risks. In fact, even the serious side effect in 1,000,000 doses administered may not be justified if there is no benefit from the vaccination. In this regard, the table below is revealing. It compares the risk deriving from the disease with the risk linked to the use of vaccines in the light of available scientific data.

Not all adverse events are caused by vaccines, therefore they must be evaluated by expert personnel

Table 1: Risks of the diseases compared to the risks of vaccinations (by Rocco Iudici, Serena Miotto, Antonio Ferro)

Risks linked to disease

Risks linked to vaccine


Pneumonia: 1/20
Death: 1/3000


Encefalite: 1/300

Congenital rubella: 1/4 if contracted in early pregnancy
Encephalitis or severe allergic reaction: 1/1,000,000

Death: 1/20


Death: 3/100

Pertosse 5

Pneumonia: 1/8
Encephalitis: 1/20
Death: 1/20

Inconsolable crying then complete recovery: 1/100
Convulsions or shock then complete recovery: 1/1.750
Acute encephalitis: 0-10.5/1,000,000

Death: not proven

Incidence: 4.000/100,000 8
Fatality: 4-9/100.000 9,10
Hospitalisation: 1,3-4,5/100,000 11

Neurological complications:

0.4-10.1% of hospitalised patients 11
Pneumonia 5-14% cases 12
Skin superinfections: 36% of hospitalised patients 13


Healthy subject aged between 12 months and 12 years (1st dose)

Varicella-like rash: 3,8%
Pneumonia: < 1%
Febrile convulsion: < 0,1%
Serious allergic reactions: < 0,01%


Incidence: 500,000 cases worldwide 14
1–3 casi /100,000 15
Fatality: 10% 14
Complications: 25% (amputations; skin tissue loss; neurological anomalies: hemiplegia, mental retardation, epilepsy, neurological deafness; psychological consequences: post-traumatic stress disorder, depression, anxiety) 16,17

MeningococCAL VACCINE 24

Uncommun (from ≥1/1,000 to <1/100): dizziness.
Very rare (<1/10.000): parestesia, anaphylactic reaction

Pneumococcal Disease IPD

Incidence: 15-20/100,000, 25-90/100,000 in children and the elderly 18.
Fatality: pneumococcical sepsis 15-20% in adults and 30-40% in over-65s, pneumococcal meningitis 12% 18
Complications: 40% of meningitis survivors present neurological sequelae

Non IPD Pneumococcal Disease

CAP Incidence: 1.6-15/1,000 19
CAP Mortality: ranges from 5 to 15% for those hospitalised 20-45% of ICU patients, 40% in over-80s 19

Pneumococcal vaccine 25

Rare (from ≥1/10,000 to <1/1,000): hypersensitivity reactions including facial edema, dyspnoea, bronchospasm, convulsions (including febrile convulsions), rash, urticarial rash or urticaria, anaphylactic reaction, angioedema, hyporesponsive hypotonic episode, injection site urticaria, injection site pruritus, hot flushes , apnea in very premature infants.

Very rare (<1/10,000): lymphadenopathy (localised around the injection site), erythema multiforme.
Infezione da Haemophilus Influenzae (HiB) 20

Incidence of invasive disease: 1/100,000 children aged ≤ 5 years
Lethality: 3% -6%

Complications: 20% of patients who survive Hib meningitis report hearing loss and other neurological sequelae. Every year 3,000 cases of invasive pathology and 386,000 deaths occur.


Very rare (<1/10,000): allergic reactions, angioedema, hypotonic-hyporesponsive episodes, convulsions, syncope or vasovagal injection reactions, drowsiness, apnea, hives, rash, extensive swelling of injection site, hardening of injection site.

Poliomyelitis 21

Annual incidence: before the introduction of the vaccine 11.4 cases / 100,000, after OPV (Oral Polio Virus), 0.002 - 0.005 VAPP cases (paralysis associated with polio vaccine) / 100,000. In 1999 an IPV-only schedule was adopted to eliminate the few cases of VAPP.
Asymptomatic infection: 95%
Paucisymptomatic infection: (fever, weakness, headache, nausea, flu-like syndrome, nuchal / spinal stiffness, pain in the limbs, often resolved completely): 4-8%
Permanent paralysis: 1%
Mortality: 5% -15% of cases of acute paralytic poliomyelitis


Very common (≥ 1/10): Local reactions at the injection site (pain, redness, induration, edema) 27

VAPP (polio vaccine-associated paralysis): 1 / 2.4 million OPV doses, not possible with IPV 21

Hepatitis B 22

Incidence: 1.29 (EU) –1.5 (USA) / 100,000 people
Mortality from acute hepatitis: 2%
Chronicization: > 30% children, <5% adults
Post chronic complications: liver cirrhosis 25%, liver cancer 5%

Hepatitis B vaccine 28

Rare (≥1 / 10,000 to <1 / 1,000): lymphadenopathy, arthralgia, paresthesia, urticaria, pruritus and rash.
Very rare (<1 / 10,000): >> Post-marketing surveillance: thrombocytopenia, encephalitis, encephalopathy, convulsions, paralysis, neuritis, neuropathy, hypoesthesia, apnea in very premature infants (≤ the 28 weeks of gestation), erythema multiforme, edema angioneurotic, lichen planus, arthritis, muscle weakness, meningitis, vasculitis, hypotension, anaphylaxis, allergic reactions including anaphylactoid reactions and serum-like illness syndrome.

Prevention and surveillance of reactions

Should adverse events occur after vaccination, they are recorded and evaluated through a system of reporting. In Italy the system of detecting adverse reactions is managed by the pharmacovigilance authority, the Italian Drug Agency - AIFA. Suspected adverse reactions to vaccines are reported through specific forms filled in by the vaccination staff, or by the doctor who is consulted regarding the reaction, and sent to the national database. Pharmacovigilance has the task of monitoring the progress of the reports and adopting any measures to guarantee the safety of the vaccinations. For this purpose, in all vaccination services, a standardised procedure has been adopted to take the pre-vaccination history, allowing health personnel, through specific questions, to verify suitability for vaccination. Experience has shown worldwide that it is not necessary to carry out tests before admission to vaccination, as anamnesis has proved to be the most important tool for assessing suitability for vaccination. In addition, the child to be vaccinated is periodically subjected to paediatric checks to ascertain the regular growth and health status. In the event of doubt, the administration of the vaccine is postponed pending appropriate investigations.

In this regard, "Canale Verde", the regional surveillance system of adverse reactions in the Region of Veneto, which integrates the national surveillance system, has very relevant data. During the regional surveillance of adverse events, in the period 1993-2011 6,119 notification cards were evaluated and classified, described in 15 activity reports. The most numerous reportings concern local reactions at the injection site (2,806 - 25% of events), followed by fever (2,069 cases - 18% of events). From Canale Verde’s evaluation, 5,715 reports (93.4%) were judged to be linked to vaccinations, while the remaining 404 (6.6%) were considered unreliable or not classifiable, respectively due to the presence of other causes or the absence of minimum elements of attribution, or lack of essential data.

The relatable adverse events were mild in 60%, relevant in 34% and severe in 6% of cases. In the long period examined, a total of 348 adverse events were considered serious, including 72 reactions at the injection site and 276 generalised events; a certain correlation with vaccination was found only in 27% of cases. Most of the manifestations resolved with complete recovery; in 21 cases, equal to 1 out of about 1,250,000 doses of vaccine, symptoms were still present after some time or were being treated at the time of data update. The average overall incidence of adverse events reported in the 1993-2011 period in the Veneto Region was 2.35 per / 10,000 doses administered; for serious events the rate is 1.3 to 100,000 doses of vaccine.

Data source: Canale Verde - Veneto Region

5,715 related events:

  • 60% slight
  • 34% relevant
  • 6% serious

Total reporting rate:

  • 2.35 / 10,000 doses of vaccine

Non guariti o in trattamento:

  • 21 cases out of over 26 million vaccine doses
  • 1 in approximately 1,250,000 doses of vaccine


The vaccines currently available for the prevention of infectious diseases are extremely safe as they are rigorously tested before being put on the market. However, like all medicines, vaccines can also give rise to generally mild adverse reactions (e.g. fever, swelling at the injection site), and to only very rarely serious reactions. Through the accurate collection of information on the candidate for vaccination, it is possible to ascertain any contraindications and therefore postpone or suspend the administration in order to reduce the risk of reactions. Surveillance of vaccine reactions through the evaluation of reportings allows us to identify rare and unknown adverse events and take appropriate measures to ensure the safety of vaccinations in the population. Vaccines are constantly monitored even during their use, resulting in some of the most stringently-controlled drugs. The risks, however modest, linked to the use of vaccines must always be compared with their benefits. Vaccines help prevent thousands of cases of illness and, consequently, related complications and deaths.

Sources / Bibliography
  9. Rawson H, Crampin A, Noah N. Deaths from chickenpox in England and Wales 1995e1997: analysis of routine mortality data. BMJ 2001 Nov 10;323(7321):1091e3
  10. Brisson M, Edmunds WJ. Epidemiology of varicella-zoster virus in England and Wales. J Med Virol 2003;70(Suppl. 1):S9e14
  11. Bonanni P, Breuer J, Gershon A, Gershon M, Hryniewicz W, Papaevangelou V, Rentier B, Rümke H, Sadzot-Delvaux C, Senterre J, Weil-Olivier C, Wutzler P. Varicella vaccination in Europe - taking the practical approach. BMC Med. 2009 May 28;7:26. doi: 10.1186/1741-7015-7-26. Review. PubMed PMID: 19476611; PubMed Central PMCID: PMC2697173
  12. Nathwani D, Maclean A, Conway S, Carrington D. Varicella infections in pregnancy and the newborn. A review prepared for the UK Advisory Group on Chickenpox on behalf of the British Society for the Study of Infection. J Infect 1998 Jan; 36(Suppl. 1):59e71
  13. Petaros P, Marchetti F. La Varicella. Medico e Bambino 2005;24:89-98
  14. Centers for Diseases Control and Prevention. Meningococcal disease: technical and clinical information (
  15. Harrison LH, Trotter CL, Ramsay ME. Global epidemiology of meningococcal disease. Vaccine 2009;24:B51–63
  16. Wright C. Counting the cost of meningitis. Presented at the Meningitis Research Foundation Conference 2011. November 8th/9th 2011, London, UK: Royal Society of Medicine
  17. Garralda ME, Gledhill J, Nadel S, et al. Longer-term psychiatric adjustment of children and parents after meningococcal disease. Pediatr Crit Care Med 2009;10:675–80
  18. World Health Organization (WHO). Estimates of disease burden and cost-effectiveness. Geneva, 2008 ( immunization_monitoring/burden/estimates_burden/en/index.html)
  19. Welte T, Torres A, Nathwani D. Clinical and economic burden of community-acquired pneumonia among adults in Europe
  20. WHO Position Paper on Haemophilus influenza type b conjugate vaccines

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