Fewer cases of diseases associated with Vaping illness have been reported in the United States, but health officials say they are not sure how to interpret it.

An additional 125 cases were reported last week, the Centers for Disease Control and Prevention (CDC) said Thursday, bringing the total number of cases this year to 1,604. Among them are 34 deaths, one more than last week.

The outbreak is still ongoing, but the new cases have decreased for three consecutive weeks. A spokeswoman for the CDC said one explanation could be a delay in reporting them.

The outbreak appears to have begun in March. No specific ingredient, electronic cigarette or vaping device has been linked to the disease. Most of those who have gotten sick said they have vaping products with tetrahydrocannabinol (THC), the psychoactive ingredient in marijuana.