One apparent result of the measures cruise lines have taken against covid-19: Outbreaks of gastrointestinal illness have been far lower than in pre-pandemic years.

So far this year, cruise lines have reported two outbreaks of vomiting and diarrhea to the Centers for Disease Control and Prevention that affected 3% or more of passengers or crew. That’s part of the outbreak threshold that determines whether the agency discloses episodes to the public. Ships must have a foreign itinerary with U.S. ports and need to be carrying at least 100 people.

The two outbreaks, affecting a total of 113 people, took place on a Carnival Cruise Line ship in late May and a luxury Seabourn voyage from late April through May. Norovirus was the cause of the Carnival illnesses, the CDC said on its site dedicated to updates about gastrointestinal illness on cruise ships; the Seabourn cause was unknown.

Last year, when cruise lines gradually started sailing again after a pause of more than a year, only one outbreak was reported. That one, caused by Vibrio and E. coli bacteria, hit 120 people on a Viking Ocean ship. Operators didn’t report any cases in 2020; the industry voluntary shut down in March of that year.

Three outbreaks in the course of more than a year is much lower than pre-pandemic numbers. In an email, the CDC said the decrease in the number of acute gastroenteritis outbreaks could “most likely be attributed” to the combination of fewer passengers on ships during that time and “nonpharmaceutical interventions used by cruise ships to mitigate COVID-19 transmission, such as increased cleaning and disinfection, increase in the number of hand sanitizer stations, crew served buffets, and physical distancing.”

Norovirus cases on land are also lower than normal. According to the CDC, the number of outbreaks reported by states during the 2021-2022 seasonal year (August to July) is lower than the range reported during the same time over the eight previous years.

The CDC’s Vessel Sanitation Program has worked with the industry to monitor gastrointestinal illness since 1975, after “an excessive number” of outbreaks. While incidence rates of stomach illness on cruise ships dropped between 2006 and 2019, according to the CDC, pre-pandemic numbers were closer to 10 or 11 outbreaks a year. Between 2017 and 2019, cruise lines reported a total of 32 outbreaks that sickened 3,359 people. Norovirus - which the CDC says “can spread quickly in closed and semi-enclosed environments such as cruise ships” - was found to be a cause in 22 of those episodes.

That’s a small fraction of the number of cruise passengers during those years: More than 13.7 million cruise passengers took cruises from U.S. ports in 2019, according to the Cruise Lines International Association.

William Schaffner, a professor of infectious diseases at Vanderbilt University School of Medicine in Nashville, said that the reduced number of cruises as the industry restarted and the lower concentration of passengers - at least on many lines - would be likely factors in the lower number of outbreaks. But he said covid-19 precautions are “undoubtedly useful” to combating the spread of norovirus, as well.

“The more rigorous we are with all of these hygienic measures, there’s spillover to other infectious agents,” he said.

Norovirus, he said, is “extraordinarily” contagious. Schaffner praised cruise lines for their disinfection practices even before the coronavirus emerged.

“Before that happened, the cruise industry under the guidance of the CDC put into place many infection-control activities that they really implemented rigorously,” he said.

Cruise lines have touted enhanced cleaning and sanitation protocols since the pandemic began. Royal Caribbean International, for example, says on its website that while hand-sanitizing stations have always been on board, the line has increased the number by 75%.

“And we’re placing them anywhere you’re most likely to use them, near elevators and at exits and entrances to all venues, plus anywhere onboard that doesn’t have handwashing stations or restroom sinks in the immediate area,” the site says.

The company said it had enhanced its cleaning protocols, noting that areas that get high traffic such as elevators, stairways, escalators and promenades get cleaned every two hours, and gangway rails are cleaned every 20 to 30 minutes when the vicinity is busy.

Norwegian Cruise Line said it had implemented “comprehensive enhanced cleaning and sanitation protocols,” and had a dedicated public health officer on all ships to “oversee the day-to-day sanitation and cleanliness of all public areas and accommodations.”

At least temporarily, some cruise lines got rid of self-service buffets in favor of having staff serve passengers at the food stations - though the old style has widely returned, according to the cruise news site Cruise Critic.

The CDC says that norovirus is spread by direct contact, or sharing food or utensils with someone who is infected. Outbreaks can also result from food, water or surfaces that have been contaminated. Common settings include health-care facilities, restaurants, schools or child-care facilities, and cruise ships, the agency says, though it notes that ships account for only 1% of all norovirus outbreaks.

The virus can last on surfaces for days or weeks, according to the CDC.

“Norovirus can be especially challenging to control on cruise ships because of the close living quarters, shared dining areas, and rapid turnover of passengers,” the CDC says on its website. “When the ship docks, norovirus can be brought on board in contaminated food or water or by passengers who were infected while ashore.”