How Google Flu Trends Works

It’s a natural part of living in the information age: You start to feel sick, so you Google your symptoms.
It’s a natural part of living in the information age: You start to feel sick, so you Google your symptoms.

Both the common cold and the flu will make you feel miserable, and because both are respiratory infections with similar symptoms — coughing, aching, headache, you know the drill — it can be difficult to know which one has you in its grip.

Every year, anywhere from five to 20 percent of the U.S. population will get the flu, mostly during that wintery stretch between December and February, give or take a few months [source: CDC]. While many sufferers will find relief in over-the-counter medicines, influenza can be serious. Influenza-related complications may require hospitalization, and sometimes complications can be fatal. Influenza, together with pneumonia (both are lower respiratory infections), ranked as the eighth leading cause of death in the U.S. in 2010, and respiratory infections were the third leading cause of deaths worldwide that year (as many as 3.2 million people) [source: CDC, CNBC].

Because seasonal influenza can cause serious complications, the Centers for Disease Control and Prevention (CDC) monitors influenza-like illness (ILI) across the U.S., tracking and analyzing flu activity to get a good picture of the incidence rate, prevalence proportion and occurrence rate of ILI throughout the year [source: Harvard Health Publications]. For its tracking purposes, the CDC considers a fever of at least 100 degrees Fahrenheit (37.8 degrees Celsius) with a cough and/or a sore throat to be an ILI.

The CDC monitors these numbers with data collected through multiple sources, including local and state health departments, 122 public health and vital statistics offices, nearly 3,000 outpatient health care facilities, more than 270 laboratories, and reports from the FluSurv-NET surveillance system [source: CDC]. All those pieces are broken down into five categories of useable information:

  1. Viral Surveillance — laboratory reports on the number of respiratory specimens taken that week and what percentage were, in fact, confirmed flu
  2. Mortality — data on the proportion of pneumonia and influenza (P&I)-related deaths and reports of influenza-associated pediatric deaths
  3. Hospitalizations — confirmed influenza-related hospitalizations
  4. Outpatient Illness Surveillance — tracking the number of outpatient visits for ILI
  5. Geographic Spread of Illness — the estimated level of flu activity by state, which could be widespread, regional, local, sporadic or no activity

Beginning on the 40th week of the year — which is the beginning of the October to May flu season — the CDC distributes weekly influenza activity reports.

The information the CDC circulates is intended to be a snapshot of current flu trends, not specific numbers of people who caught the flu during that flu season or year. The focus is on whether flu outbreaks are occurring, where flu is being reported, when it was reported and which influenza viruses are to blame.

While the data released by the CDC provides an accurate picture of flu trends, that data, once it's compiled and analyzed, is also one to two weeks old. It can't tell you whether a new pocket of flu emerged in a specific city over the previous weekend, but it's good for measuring the overall impact of flu on the U.S. population, in addition to making flu-related public health recommendations.

For instance, by monitoring which strains of influenza were circulating in the 2014 flu season, CDC epidemiologists were able to tell with data collected between Oct. 1 and Nov. 22 that one of the three chosen strains included in that year's flu shot had mutated, and the vaccine would be less effective that season.

But what if you wanted to know more about that a flu epidemic that's spreading through a nearby city? Google would like to help with that.