The peak of norovirus season 2023 may be over, but the highly contagious stomach bug is still circulating in the United States.
According to the most recent data from the U.S. Centers for Disease Control and Prevention, about 6% of tests for the virus are still coming back positive in the U.S., which is down from a peak of 16.4% in mid-March. That said, the current percentage of tests coming back positive is still higher than in mid-July 2022, 3.6%.
Other CDC data show the 14 state health departments that are part of its NoroSTAT surveillance program reported seven norovirus outbreaks in total for the week of June 5, 2023, the most recent data available. This was the same number as what was reported the week before, and down from a peak of 62 outbreaks in February 2023. At this point in 2022, closer to zero outbreaks were reported; for the 2020 to 2021 norovirus season, the number of outbreaks being reported at this point in the year was about the same.
Dr. Luis Ostrosky, an infectious disease specialist at UTHealth Houston and Memorial Hermann in Houston, told TODAY.com in June that “it’s not unusual to be see (norovirus) cases in April and May, (and) we are continuing to see many cases.”
What's more, norovirus outbreaks on cruise ships are at their highest in 11 years, according to CDC data. There have been 13 recorded so far this year. Most recently, norovirus swept through a Viking Neptune cruise ship, sickening 110 people, about 13% of the ship's total guests. Other cruise lines that have been impacted by this year's norovirus season include Celebrity Cruises, Princess Cruises and Royal Caribbean International.
In 2022, only four norovirus outbreaks were reported on cruises. In 2021, there was only one, and there were zero reported in 2020. Before the pandemic, there were 10 reported in 2019 and 12 in 2018.
Norovirus — commonly known as the “stomach flu” — is actually a group of viruses that cause acute gastroenteritis, an inflammation of the lining of the stomach and intestines, which leads to acute vomiting and diarrhea, per the CDC. Unlike its nickname suggests, norovirus is not related to the flu or influenza viruses.
Why did norovirus surge this year?
Norovirus outbreaks are common in the U.S., Kate Grusich, CDC spokesperson, told TODAY.com in a statement in February. Each year, norovirus causes 19 to 21 million cases of vomiting and diarrhea, 109,000 hospitalizations and 900 deaths, per the CDC.
Although norovirus can spread year-round, it has a wintertime seasonality in the U.S., said Grusich, so cases tend to peak during the colder months. The vast majority of outbreaks occur between November and April, according to the CDC.
The CDC data show that norovirus cases peaked between February and March of 2023. One CDC chart shows that cases during the peak of the 2023 norovirus season were lower than they were in 2022 but higher than in 2021.
Dr. Ali Alhassani, a pediatrician at Boston Children’s Hospital and head of clinical at Summer Health, told TODAY.com in February that norovirus cases at the time were going up "quickly" and it started "pretty suddenly."
Ostrosky also told TODAY.com back in February that “norovirus activity (was) higher and earlier than usual, but definitely not a big outlier compared to pre-pandemic levels.”
The reason for the surge may be due to children who managed to evade norovirus over the past few years who were then exposed to norovirus and many other viruses that tend to peak and circulate among schools in the winter and early spring.
"We've always had seasonal increases and waves of norovirus, and our hospitals would be filled with kids," Dr. Albert Ko, infectious disease physician and professor of public health, epidemiology and medicine at Yale School of Public Health, tells TODAY.com. "What kind of took us off was the pandemic," Ko adds.
Prevention measures implemented to curb COVID-19 were likely effective in preventing norovirus outbreaks, said Grusich, and as restrictions have relaxed, the number of outbreaks returned to levels similar to pre-pandemic years.
How does norovirus spread?
Norovirus is transmitted primarily "when bacteria or viruses shed in stool ends up on our hands and surfaces and then eventually ends up in our mouth and we ingest it and get infected," said Alhassani.
Norovirus may be transmitted directly from an infected person or from contaminated surfaces, objects, foods or drinks.
"Anybody who is in close contact with someone who has an active infection with norovirus is at high risk of getting it," said Alhassani. Norovirus can spread through activities like caring for an infected person, changing diapers or sharing utensils.
"Norovirus is so infectious that even if somebody throws up and there's droplets of vomit aerosolized in the air, that can actually cause infection," Alhassani added.
It takes a very small number of virus particles to transmit the disease, said Ko, which is why norovirus causes so many explosive outbreaks. Per the CDC, less than 100 norovirus particles can make you sick, and infected people typically shed billions of particles.
Most people are infectious from the time symptoms begin until about two or three days after symptoms resolve, Ko said, but some people can remain contagious or up to two weeks after recovery.
Outbreaks often occur in schools, day cares, nursing homes and cruise ships, the experts noted.
What are the symptoms of norovirus?
The most common symptoms of norovirus are vomiting, nausea, diarrhea and abdominal pain, said Ko. Other possible symptoms include a headache, body aches and a low-grade fever. Norovirus symptoms usually develop within 12 to 48 hours after exposure, per the CDC.
"Norovirus ... will just last a few days," said Ostrosky. "For the majority of the population, it’s going to be just a nuisance."
Those at higher risk of developing severe or prolonged symptoms include babies, the elderly and the immunocompromised, said Ostrosky. If symptoms transition into chronic diarrhea and weight loss, this can lead to complications like dehydration or poor absorption of medications, he added.
What is the treatment for norovirus?
"There's actually no specific treatment or antiviral for norovirus," said Ostrosky. Hydration is key to replenish fluids lost from vomiting and diarrhea, the experts noted, which means drinking plenty of water, Pedialyte or sports drinks.
"Then it's just eating bland foods and trying to let it pass through the body, which usually takes like one to three days," said Alhassani, adding that over-the-counter anti-nausea medicine and pain relievers may also be used to ease symptoms.
“The vast majority of people can be managed at home and, in fact, should be isolated at home until they’re improving, given how contagious norovirus can be,” said Ostrosky.
However, it's important to watch for signs of severe dehydration and to contact a health care provider if these occur, the experts noted. These include dry mouth, decreased urination, dizziness and, in children specifically, crying without tears, fussiness or unusual sleepiness, per the CDC.
Children under 1, people who are immunocompromised, or those with prolonged or severe symptoms should also be seen by a physician, said Alhassani. “While it infects many people, (norovirus) tends to not send as many people into the hospital and certainly the ICU,” he added.
A person can be infected with norovirus multiple times in their lifetime. After recovering, you may possibly develop some short-term immunity, said Ko, but it won't be robust and wanes quickly.
“It's only partial immunity ... because there are different types of norovirus, and being exposed to one doesn’t give you complete protection to another,” he explained.
To prevent norovirus infection, don't make this mistake
There's no vaccine against norovirus, said Ko, but there are steps you can take to prevent infection and transmission.
Hand hygiene is extremely important — but the way you clean your hands matters, Ostrosky noted, and it has to be with soap and water. Hand sanitizer does not work against norovirus.
“Norovirus is one of the few viruses that doesn’t get deactivated by alcohol. You actually need to use soap and water to physically destroy it and remove it from your hands,” said Ostrosky.
Wash your hands after using the restroom, before eating or cooking and after caring for someone with norovirus.
When cleaning surfaces or objects that may be contaminated with norovirus, Ostrosky suggested using a high-level disinfectant like bleach.
If you or your child are sick with norovirus, isolate to prevent the virus from spreading within the household, said Alhassani. Anyone sick with norovirus should stay home until they feel better. “Avoid food preparation until at least 48 hours after symptoms stop,” said Grusich.
"We can expect to continue seeing more viral illnesses, both respiratory and gastrointestinal, in this post-COVID era we're sort of approaching," said Ostrosky, adding that the basics of hand-washing, isolation and respiratory etiquette can go a long way.