Your child has an allergic reaction to nuts, strawberries or a bee sting that leads to anaphylaxis, a life-threatening medical emergency. That's scary enough. But some children have a second reaction hours later. That repeat attack can occur even if they’ve been treated and aren't re-exposed to the substance that triggered the initial attack.

The worse the first allergic reaction, the more likely a follow-up reaction, a new study found.

Anaphylaxis symptoms can occur very suddenly, minutes after exposure to an allergen. Initial symptoms such as a runny nose or rash can lead quickly to much more serious ones, including difficulty breathing, hives, a tightening of the throat and heart palpitations.

Related: Ban These 7 Allergens from Your Home

What the study found

The researchers evaluated the medical records of 484 children seen in the emergency departments of two Canadian hospitals for anaphylaxis. Nearly 15 percent of the kids had a delayed, second reaction.

Other studies have found that anywhere from 3 to 23 percent of people who have anaphylaxis will have a second reaction. The new study is believed to be the largest of its kind.

At least half of the second reactions were serious and required treatment with epinephrine. This drug, also known as adrenaline, works by relaxing the airways and tightening blood vessels so blood is pumped more efficiently.

Most of the children had a reaction to food, most often peanuts, says researcher Waleed Alqurashi, MD, assistant professor of emergency medicine at the University of Ottawa. Some reacted to tree nuts, milk, eggs and seafood. In the United States, food allergies affect up to 6 percent of children. Other triggers included medication, exercise and bee stings.

Besides the severity of the initial allergic reaction, other factors increased the risk of a second:

  • Delay in treatment. Waiting longer than 90 minutes to get emergency care upped the chances of a second reaction, even if the initial symptoms were minor.
  • Age. Children ages 6 to 9 were more than three times as likely to have a severe second reaction.
  • Initial treatment. If children needed more than one dose of injected epinephrine or received an inhaled beta-agonist, they were more likely to have a follow up reaction. This reflects the severity of the initial reaction, Alqurashi says.

Related: Hidden Food Allergens: 18 Surprising Foods (and Drinks) That Have Them

What else parents should know

Because of the possibility of a second reaction, children should be observed closely after the initial treatment. In the study, most of the second reactions occurred within six hours of the first, though some occurred 18 hours or more afterward.

If your child has a known severe allergy, his doctor can prescribe epinephrine in auto injection form to be used immediately after symptoms of a severe reaction occur. "Early administration of epinephrine is really the key message," Alqurashi says. Most of the reactions in the study occurred at home.

Initial treatment must be followed immediately by emergency care. After that, the child should be seen by his regular doctor for evaluation.

Symptoms of anaphylaxis, according to the American College of Allergy, Asthma and Immunology, include:

  • Breathing difficulty
  • Hives or swelling
  • Throat tightness
  • Hoarse voice
  • Nausea, vomiting, abdominal pain, diarrhea
  • Low blood pressure, dizziness, fainting
  • Rapid heart beat
  • A feeling of doom
  • Cardiac arrest

Kathleen Doheny is a Los Angeles journalist specializing in health, behavior and fitness topics.