Tip Sheets

When reporting on surveillance programs, look for the denominator

By Tara Haelle

Surveillance is the process or system for tracking cases of risk factors, medical conditions, disease cases, adverse events, etc. Journalists often must rely on surveillance programs to report on outbreaks or side effects or in doing investigative research, but it’s important to know what kind of surveillance you’re using.

Surveillance is often used to track incidence of a disease, such as keeping up with where measles cases are during an outbreak or where remaining polio cases are in the attempt to eradication the disease. It’s also used to track prevalence, such as the total number of women living with breast cancer, or adverse events, such as tracking hospital-acquired infections or possible side effects from vaccines or drugs that the FDA has already licensed.

The two basic types of surveillance are active and passive. Understanding the difference is essential since passive surveillance systems cannot be used or interpreted in the same way as active surveillance systems.

Passive surveillance is the collection of data from those who voluntarily report it, whether it’s hospitals, individual health care providers, parents, health departments or another source. The information is collected passively and retroactively — no one is actively systematically looking for specific cases or collecting data on every individual in a particular setting.

Passive surveillance is useful for looking for patterns or “signals,” such as a cluster of disease cases that might indicate the start of an outbreak or a cluster of unexpected adverse events, such as a higher-than-anticipated number of strokes reported by people taking a certain drug when stroke was not among the medication’s known risks. However, a major limitation of passive surveillance is that no denominator exists. That is, there is no way to know how representative the reported cases are because there’s no way to know what the baseline population is. You don’t know how many people did not catch the disease or experience the adverse event.

Active surveillance involves setting up a system to actively search for cases with a reporting system or with a systematic protocol, such as calling every health department or all the clinics in an area during a disease outbreak or tracking all adverse events from every person in a hospital system who receives a specific vaccine. Active surveillance has a clear denominator: the total number of patients seen at the clinics you call is the baseline population, and you can establish prevalence or incidence based on how many of those are cases.

An example of why it’s so important to understand the difference are two types of vaccine adverse event surveillance programs: the Vaccine Adverse Event Reporting System (VAERS) and Vaccine Safety Datalink (VSD). VAERS collects any report of an adverse event that follows administration of a vaccine: parents or health care providers can report the events, and the report remains regardless of the report’s veracity or whether the adverse event resulted from the vaccine or not. (A doctor once proved this point by submitting a VAERS report that he turned into the Incredible Hulk after receiving a vaccine; the report stayed until he gave permission to remove it.) VAERS cannot be used to determine the frequency of adverse events from vaccines or even whether vaccines can or cannot cause certain side effects.

The VSD, on the other hand, is a collection of eight health care organizations who actively track adverse events that occur following vaccination. Vaccine administration and any subsequent adverse events are all explicitly documented in the patient’s chart with the knowledge that those charts could be used to compare adverse events and vaccine administration. The denominator — the number of patients receiving the vaccine — is definitive, so it’s possible to analyze whether a certain adverse event is associated with the vaccine or not.

If someone relies on VAERS to claim that certain side effects occurred from a vaccine, the report is not reliable. VAERS can only be used to look for an uptick in a certain report. But researchers have used VSD since 1994 to actively look for associations between vaccines and certain adverse events. For example, it was because of VAERS that public health officials noticed a higher number of reports of intussusception that they might expect after children received the rotavirus vaccine. But they needed to use VSD to conduct several studies and find out whether the number of intussusception cases after the rotavirus vaccine was similar to the background rate or if there was actually a statistically higher number occurring in the days or weeks after the rotavirus vaccine.

The FDA Adverse Event Reporting System (FAERS) is similarly a passive surveillance system, so the presence of a report—or even many reports — does not mean the drug had the effect being reported. It means a study needs to be conducted to determine whether an association exists. When women or their doctors reported many problems with the permanent contraception device Essure, the high number of reports did not mean Essure definitely caused all of those problems, but it did mean it was important to conduct a study to find out if an association existed.