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Airborne Transmission of Norovirus

In 2015, doctors participated in a study which determined that norovirus can be transmitted in an airborne form. They published their findings in an article titled Detection and Quantification of Airborne Norovirus During Outbreaks in Healthcare Facilities in the highly respected journal of Clinical Infectious Diseases.

The medical experts concluded that rules need to be reviewed to take into account the possibility of airborne transmission of norovirus. They suggested the use of “mobile air-filtration units” or the “wearing of respiratory protection around patients with gastroenteritis” should be considered.

In May of 2015, a reporter for HealthDay News / U.S. News and World Report wrote an article titled Cruise Ship’ Norovirus Bug Can Spread by Air, Study Finds, saying that “notorious bugs that have infected scores of people and ruined countless cruise ship vacations — can spread through the air and infect people several feet away, according to new research.”  Thereafter, I wrote a series of articles asking whether cruise lines would be investigating the need to equip their ships with filters or require crew members responding to norovirus outbreaks to be equipped with respirators.

Cruise Lines Always Blame the Guests’ Hygiene

The cruise industry’s lobbying team at the Cruise Line International Association (CLIA) has been lecturing the public for decades that passengers who came on the ship while ill or didn’t wash their hands were usually the cause of the virus’ spread.  Of course, norovirus can be brought on board ships by an ill passenger as well as by crew members. Gastro-related illness can also be caused by contaminated food and/or water, which the Centers for Disease Control (CDC) and the Federal Drug Administration (FDA) have long stated is the most likely cause of the virus. Norovirus outbreaks can also be caused by crew members working while infected and poor cleaning practices. Of course, it takes an investigation by a skilled team of epidemiologists to come to a final, scientific explanation.

I wondered how the industry would spin the scientific data proving that norovirus can spread in the air and, by deduction, then easily be transmitted throughout the ship by the cruise ship’s air-conditioning.

No Cruise Line Investigated Airborne Transmission of Viruses

No travel related publications or any major news organization for the last five years covered the issue of whether norovirus outbreaks could be explained, at least partially, by airborne transmission. From the date of the 2015 study to the beginning of the coronavirus pandemic in 2020, there have been no announcements by CLIA or any of the individual cruise lines regarding the transmission of airborne viruses on ships or any consideration given to using sophisticated air filters for the shipboard air conditioning systems.

Coronavirus – Transmission of Droplets Through Close Person-to-Person Contact and Airborne Transmission?

Turning from norovirus to coronavirus, at the beginning of the coronavirus outbreak on the Diamond Princess, I asked Princess Cruises whether the virus can also be spread through airborne particles. In response to my inquiry which I posted on Twitter (along with a photo of Princess crew members huddled together in a hallway in the crew area), Princess Cruises denied that the virus can be transmitted through the air. It cited to the World Health Organization (WHO) for the proposition that the virus is “mainly” transmitted via droplets from close person to person contact.

A New Study – COVID-19 Likely Spread on the Diamond  Princess Via the Air 

Yesterday SFGate published an article (not peer reviewed yet) titled Study: Ship’s AC system likely spread COVID-19 which contradicts Princess Cruises’ claim that the virus was spread on the Diamond Princess (photo top) via droplets from close person-to-person contact. A new, unpublished study of data collected during the COVID-19 outbreak on the Diamond Princess in February suggests that the virus likely spread through the ship’s air-conditioning systems. Of course, the crew members were most affected by the virus as they were in cabins with no windows or balconies without fresh air.

A prior unpublished study (also not peer reviewed) dated three months earlier reached a different result, namely that the ship’s central air conditioning system did not play a role in the the long-range airborne transmission of the virus. There have been peer reviewed published studies indicating that HVAC as a major source for indoor and environmental contamination that can explain the swift viral spread of COVID-19.  There has also been studies regarding small droplet aerosols in poorly ventilated spaces and COVID-19 transmission. The Washington Post published an article We Cannot Keep Ignoring the Possibility of Airborne Transmission which discussed public health expert Joseph Allen’s opinions:

“I’ve been warning about airborne transmission of covid-19 since early February. The explosive transmission on the Diamond Princess cruise ship, as well as other coronavirus outbreaks, constituted telltale signs that airborne transmission was happening. Close contact transmission was likely happening on that cruise ship, but the disease had spread far more quickly than non-airborne diseases typically spread.”

There has been mixed opinions about whether coronavirus can be transmitted in an airborne manner. Obviously, it is not a binary explanation (droplets versus airborne). Clearly, there have been an increasing realization that coronavirus “may” be transmitted through the air.  Last week, over two hundred experts from over 30 countries wrote an open letter to the WHO, which appeared in the journal of Clinical Infectious Diseases, suggesting that coronavirus lingers in the air indoors and floating infectious particles may sicken others nearby. The New York Times addressed this issue in an article titled 239 Experts With One Big Claim: The Coronavirus Is Airborne. WHO responded to the increasing scientific research; last week the agency acknowledged that airborne transmission of the coronavirus may cause infections in indoor spaces. The WHO says aerosol Covid-19 transmission “cannot be ruled out.”

Some Experts Recommend HEPA Filters on the Ships 

Even before this newest report was published last week, some air quality experts concluded that cruise ship air conditioning systems are not designed to filter out particles as small as the coronavirus, allowing the disease to rapidly circulate to other cabins. Cruise ships typically do a poor job of filtering air, although there has been no clear consensus that air-conditioning systems can transmit coronavirus. Many experts nonetheless recommend the installation and use on cruise ships of medical-grade HEPA (high-efficiency particulate air) filters to catch and remove the majority of pathogens.

In the past twenty years whenever a norovirus outbreak ocurrs on a ship, the cruise line always blames the passengers, usually before there is even an opportunity to conduct an epidemiology assessment. Cruise lines do not send epidemiologists onto cruise ships following an outbreak. They do not test whether the outbreak was caused by contaminated food or water versus poor hygienne by a guest or crew member. In the over 125 disease outbreaks at sea on cruise ships in the last ten years, the cruise lines have never come to a single determination as to the cause of an outbreak. However, they usually intimate that such incidents could be avoided simply by the guests washing their hands.

Why Didn’t Cruise Ships Install HEPA Filters?

By historically blaming the passengers, and never accepting or even considering the possibility that its food may be contaminated or its crew members (particularly food handlers) may be ill, the cruise industry has never invested in investigating the true cause of disease outbreaks over the years on their cruise ships. There have been literally hundreds of disease outbreaks in the last two decades, involving not only norovirus but e-coli, measles, Legionnaires’s Disease and other exotic viruses.

Yet, none of the cruise lines have ever hired a staff of epidemiologists, scientists,  public health doctors and infectious disease experts to study disease outbreaks at sea. Why? Because it seems that the cruise companies felt it unnecessary or too expensive. Before the pandemic, cruise lines responded to disease outbreaks like they did any other unpleasant occurence at sea – by always denying responsibility or blaming their guests. The cruise lines’ response has been to involve their PR teams to spin the story rather than to hire scientists to understand and develop solutions to the problem.

So far, NCL seems to be the only large major cruise line which is considering the possibility of using more effective filters in its air conditioning systems. Last month, Travel Pulse reported that NCL is considering the replacement of existing air filters throughout it fleet of ships with HEPA filters. NCL’s Norwegian Peace of Mind Sail Safe New Enhanced Health & Safety Protocols state that NCL will install “medical-grade air-filters, H13 HEPA, that remove 99.95% of airborne pathogens across our entire fleet to ensure the air you breathe is clean.”  Whether this will really occur is less than certain.

Travel Weekly today reports that Windstar Cruises will retrofit its six small ships with hospital-grade air filters.  The cruise line working group which created what is commonly called the new EU protocols has a chapter ( “7.8 – Adequate ventilation”). It suggests that the “ventilation rate should be such as to provide as much outside air as possible.” It also proposes (but does not require) that air handling units be switched from recirculation to 100% outside air whenever possible or cruise ships should explore the possibility of “improving air filtration as much as possible and using HEPA filters or Ultraviolet Germicidal Irradiation (UVGI).”

It remains to be seen whether other major cruise lines will follow suit. Carnival Corporation, for example, has over 90 large cruise ships which should be installed with HEPA filters. So far, Carnival has not announced any new protocols. Its CEO Arnold Donald stated at the recent earnings call that the company has “not actually gotten to the point of serious resumption of cruise discussions with the CDC . . . ”

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Update: A reader brought to my attention Virgin Voyages’ “Voyage Well” policies which tout “a breath of (literal) clean air.” Virgian claims that its ships have installed “the latest technology from AtmosAir Solutions — an air purification system that disinfects air on board. Leveraging bi-polar Ionization technology, this air purification system has been shown to kill 99.9% of viruses — making us the first in our industry to treat 100% of the air on board with this technology.” Plus it states that “our sea terrace ratio is one of the highest in the industry — allowing plenty of access to (salt-infused) fresh air.”

Photo credit: Diamond Princess – Alpsdake – file extracted from another file: Diamond Princess (ship, 2004) and Port of Toba.jpg, CC BY-SA 4.0, commons / wikimedia; Diamond Princess bottom – © Carl Court/Getty Images via Live Science Coronavirus quarantine ends for some cruise ship passengers, sparking worries over virus spread.

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