These analyses suggest that ED visits associated with EVALI began to spike in June 2019, peaked with cases reported to the CDC in September, and have decreased since then. From January 2017 to June 2019, there was a gradual increase in the incidence of ED visits that involved mentions of e-cigarette product use. In the first week of June 2019, the incidence of reasons for the visit that included mention of these products and shortness of breath began to increase sharply, which suggests that this EVALI-related symptom was appearing more often in persons using e-cigarette products. Also in early June, the incidence of discharge diagnoses with codes that the CDC would later recommend for EVALI began to increase nationwide among persons 11 to 34 years of age. These increases that occurred in June 2019 coincided with the first known cluster of EVALI cases.1 As with cases reported to the CDC2 and a previous analysis in Illinois,1 the ED visit rates were higher among male patients than among female patients. This analysis of ED data suggests that the incidence of EVALI increased sharply in the summer of 2019 and had not been occurring at the same level over a longer period without detection.

There are many exposures that might have contributed to the gradual increase in the incidence of a report of e-cigarette product use as the reason for the visit during the period from January 1, 2017, to June 1, 2019, especially among patients 10 to 19 years of age. The ED visits could include sporadic cases from the same products or substances that later contributed to the wider outbreak when they became more commonly used. Another possibility is the increasing use of e-cigarettes containing nicotine. The percentage of U.S. high school students reporting e-cigarette use in the previous 30 days increased from 11.7% in 2017 to 27.5% in 2019.16 Newer-generation e-cigarettes use nicotine salts,17 which allow more nicotine to be inhaled with less irritation.18 Nicotine salts may increase the risk of adverse effects to the lungs owing to more frequent and stronger inhalation or may increase the risks of other health effects, such as nicotine poisoning or effects from other potential toxicants such as flavoring agents. The rise also coincided with the increased use of cannabinoid (CBD) oil in these products.19 The increase in the incidence of these ED visits may also be driven by acute intoxication from THC (which increased in availability over this time period), synthetic cannabinoids, or other drugs. Because the causes of the earlier increase are not yet known, more study is needed to understand the increase between January 2017 and June 2019.

The number of ED visits with e-cigarette product use mentioned in the reasons for the visit peaked during the week of September 8, 2019, which was the same week as a peak in Google searches for similar terms, after there was widespread national media attention about this issue. This spike could reflect a real increase in EVALI cases over time. Results for the outcome query regarding lung injury, which relied on the use of specific diagnostic codes, increased in early June 2019 and peaked during the week of September 29, 2019, before the CDC recommended these codes for use on October 17. The slower increase and later peak with the outcome query than with the exposure query might have been caused by more gradual uptake of certain diagnostic codes. Alternatively, some of the ED visits with e-cigarette product use mentioned in the reason for the visit but without an EVALI-related diagnosis may represent less severe effects or were early signs of more serious injury, such as shortness of breath that later progressed to respiratory failure.

This study has limitations. First, both the exposure query and the outcome query returned records that were unrelated to the current injury and in which e-cigarette use was incidental to the visit. Second, the exposure query was probably affected by public and clinical awareness of the outbreak, which increased the likelihood that e-cigarette products would be mentioned in the reasons given for the visit. Third, the outcome query relied on the use of specific codes and probably underestimated the number of visits for EVALI. The magnitude of the underestimate cannot be assessed at the national level. In Illinois, an analysis of confirmed and probable cases showed that 23% of 159 patients 11 to 34 years of age seen in an ED that participated in syndromic surveillance had a CDC-recommended discharge diagnosis, so these cases would have been captured by the outcome query. Finally, NSSP coverage is not uniform across or within all states, and health care facilities contributing data to the system change over time as new hospitals are added to the system and, more rarely, when they close. From the first week of June 2019 through the first week of November 2019, the number of facilities sending data to NSSP increased from 3109 to 3247.

Despite these limitations, syndromic surveillance also has important strengths, including timeliness, automated reporting, the ability to examine local and national trends rapidly, and the flexibility to change syndrome definitions rapidly without changing data collection, which make ED data a valuable complement to traditional epidemiologic investigations. In the future, it may be possible to use ED data in combination with reports from clinicians to understand any spikes that may be caused by increasing exposure to e-cigarette products. The CDC announced in November 2019 that it would no longer request EVALI case reports for patients who were not hospitalized; data from ED visits that do not lead to hospitalization can thus offer insight on trends in less severe cases.

ED data suggested that the incidence of EVALI increased sharply between early June and early September 2019. Although the incidence of ED visits for which reasons given for the visit and discharge diagnosis codes included terms associated with EVALI has decreased since the peak, the incidence has not decreased to the rates that were observed before June 2019; therefore, there is a need for continued monitoring and prevention.