Risks and Monitoring
Part of Vaccines
Identifying, quantifying, and managing vaccine risks while maintaining the surveillance systems that protect programs.
Why This Matters
No medical intervention is risk-free. Vaccines carry small but real risks: anaphylaxis, occasional serious adverse events, and in rare cases vaccine-associated disease. These risks must be weighed honestly against the much larger risks of vaccine-preventable disease.
What distinguishes safe vaccine programs from unsafe ones is not the absence of risk, but the presence of systems for identifying, monitoring, and responding to risk. A program that acknowledges and tracks adverse events builds trust. A program that denies all adverse events — or has no way to detect them — eventually loses credibility when events occur and no one had warned the community.
In a rebuilding context, risk monitoring is constrained by resources. But the core elements — documenting all adverse events, analyzing patterns, and adjusting practice when patterns suggest a problem — are achievable with paper and good organizational discipline.
Categorizing Vaccine Risks
Program Errors (Preventable)
The largest category of vaccine “adverse events” globally are not true biological responses to vaccine — they are errors in program implementation.
Wrong substance injected: Vaccines confused with medications, antiseptics, or other substances at vaccination stations. Prevention: clear labeling, single-substance preparation at any station, visual confirmation before every injection.
Reconstitution errors: Wrong diluent, wrong volume, improper mixing. Prevention: standardized reconstitution protocols, direct supervision of reconstitution.
Non-sterile technique: Contamination from reused needles, unsterile equipment, or injection-site preparation failures. Prevention: strict single-use needle policy or verified resterilization protocol, site preparation before injection.
Incorrect dose: Too much or too little vaccine. Prevention: standardized dose measurement, trained vaccinators, supervision.
Incorrect route: Vaccine intended for intramuscular given subcutaneous or intradermal. Prevention: clear route marking on all vaccine packages, training.
Cold chain failure: Vaccine given after heat exposure renders it inactive. Prevention: cold chain monitoring, vaccine vial monitors, discard policy for compromised vials.
These errors account for the majority of serious adverse events in field vaccine programs worldwide. They are all preventable with training and supervision.
Injection Reactions (Site-Related)
Local reactions from the physical act of injection, independent of vaccine content.
Fainting (vasovagal syncope): Common, particularly in adolescents and young adults. Fear and pain trigger autonomic response: brief loss of consciousness. Not dangerous unless patient falls. Prevention: vaccinate seated patients; observe after injection.
Hematoma: Bruising from needle trauma to local vessels. Expected finding; rarely concerning.
Injection-site infection: Bacterial introduction through non-sterile technique. Presents as increasing redness, warmth, pain, and swelling after day 3 (not immediate). May develop into abscess. Prevention: sterile technique. Treatment: drainage if abscess, antibiotics if spreading cellulitis.
Nerve injury: Rare. Incorrect site (too high on deltoid near shoulder) can damage axillary nerve. Incorrect buttock injection can hit sciatic nerve. Prevention: proper site identification and technique training.
Vaccine-Specific Biological Risks
True biological adverse events related to vaccine content.
Anaphylaxis: 1-2 per million doses for most vaccines. Peak risk at 15-30 minutes post-injection. Life-threatening but treatable with epinephrine. Prevention: pre-screening for known allergies; post-vaccination observation; epinephrine available at vaccination sites.
Febrile seizures: Children 6 months - 5 years are susceptible to seizures with any fever. Rate approximately 1-2 per 10,000 doses for vaccines that cause fever. Appears frightening but is rarely dangerous. No long-term neurological consequences in the vast majority. Management: fever control, observation, reassurance.
Vaccine-strain viral disease: With live attenuated vaccines, the attenuated virus may occasionally revert toward virulence or cause disease in individuals with impaired immunity.
- Oral polio vaccine (OPV): 1 case of vaccine-associated paralytic polio per 750,000 first doses
- BCG dissemination: occurs in severe combined immunodeficiency (SCID)
- Progressive vaccinia: occurs in immunocompromised individuals given vaccinia
Intussusception: A bowel obstruction that was associated with the first generation rotavirus vaccine (Rotashield, withdrawn 1999). A rare example of a biological adverse event detected through post-marketing surveillance — exactly the kind of surveillance system that makes vaccine programs safe.
Risk-Benefit Framework
Every decision to vaccinate involves comparing vaccine risk to disease risk.
Always vaccinate when:
- Disease is common in the community and causes significant mortality
- Vaccine is effective (>70% protection demonstrated)
- Vaccine adverse event rate is much lower than disease mortality or major morbidity
Consider carefully when:
- Disease is rare or mild in the specific population
- Vaccine has significant adverse event rate
- Individual has risk factors for adverse events (immunocompromise for live vaccines)
Do not vaccinate when:
- True contraindication exists (see Contraindications)
- Disease is absent from the region and introduction risk is low
- Vaccine quality cannot be verified
Quantitative example: Measles mortality in unvaccinated populations: 1-5 per 1,000 cases. Measles encephalitis: 1 per 1,000 cases. Measles vaccine serious adverse events: approximately 1 per 100,000 doses. Risk ratio: vaccinating is 100× safer than not vaccinating, even ignoring herd immunity benefits.
Surveillance System Architecture
Passive surveillance (baseline): Healthcare providers report adverse events they observe. Requires:
- Standardized reporting form
- Clear case definitions (what counts as reportable)
- Central collection point
- Regular review of submitted reports
Passive surveillance is cheap but misses most adverse events (estimated detection rate 1-10% of actual events). Useful for detecting unusual or severe events.
Active surveillance: Practitioners actively seek out adverse event information from vaccinated individuals through follow-up visits, phone contact, or community health worker home visits. More resource-intensive but detects far more events.
In a rebuilding context, active surveillance for a defined period after a new vaccine introduction (e.g., 1 month, during which every vaccinated person is contacted) can characterize the safety profile of a new product.
Signal detection: When adverse event rates exceed background expectations, investigate:
- What changed? (new vaccine lot, new vaccinator, new target population, seasonal factor)
- Are cases clustered in time and place? (suggests single-source problem like contaminated lot)
- Are cases consistent with a known adverse event profile, or novel?
- Does the rate exceed the expected background rate for this type of event?
Response to signals:
- Investigate before panicking — most signals are false alarms
- Suspend implicated vaccine lot (not entire program) while investigating
- Communicate transparently with community — information vacuums fill with rumor
- Document investigation findings regardless of outcome
- If true causal association found: adjust practice (change formulation, add contraindication, improve screening)
Communication of Risk
The problem with silence: If a community has no information about vaccine risks, any adverse event — even a coincidental one — will appear to be caused by vaccination. The resulting distrust can collapse immunization programs. Transparent communication about known risks is protective.
Language principles:
- Use absolute numbers, not only percentages: “1 in every 100,000 doses causes severe reaction” is more meaningful than “0.001%”
- Compare to disease risk: “measles kills or permanently harms 2 in every 1,000 children who get it; the vaccine causes severe reaction in 1 in 100,000 children vaccinated”
- Acknowledge uncertainty: “we don’t yet know all the risks from this new vaccine, which is why we will follow up with everyone who receives it”
- Avoid claiming vaccines are “safe” without qualification — prefer “the expected benefits are much greater than the expected risks”
Trust maintenance: Communities trust programs that:
- Disclose known adverse events before they occur
- Respond promptly and honestly when events happen
- Demonstrate that adverse events are being tracked and will be acted upon
- Show that the program is designed to protect community members, not to achieve targets at any cost
The goal of risk communication is not to minimize fear — it is to enable informed decision-making. A community that understands real risks and real benefits, and makes a collective decision to vaccinate, is far more resilient than one that was given incomplete information and acts out of compliance rather than understanding.