COVID-19 on a cruise ship – Expert Reaction

Covid-19 cases have been confirmed on one of the first cruise ships to travel around the country in more than two years.

Ovation of the Seas, carrying almost 5000 passengers and 1300 crew, sailed in to Napier on Monday from Tahiti, and arrived in Wellington this morning.

The SMC asked experts to comment.

Professor Michael Baker, Department of Public Health, University of Otago, Wellington, comments:

“The role of cruise ships in relation to Covid-19 in New Zealand has changed fundamentally since the early days of the pandemic in March 2020. At that stage New Zealand was managing its external borders very tightly to limit, and then prevent importation of Covid-19 entirely as part of its elimination strategy.

“Now that we have more than 10,000 people a day arriving on flights into New Zealand, with no requirements for Covid-19 vaccination or testing, our borders are largely open. In this context, cruise ships can only make a relatively small contribution to Covid-19 importation. Their infectious disease control protocols may now be stronger than for air travel, with passengers sometimes required to be vaccinated and test Covid-negative prior to embarking.

“The Ovation of the Seas cruise ship, which arrived in Wellington today, is carrying about 4500 crew and passengers so it is not surprising that there is transmission of Covid-19 on the ship. Infected cases are required to isolate on board, which will reduce but not prevent some of them potentially bringing infections with them when they leave the ship. From a risk assessment perspective, it would be useful if these ships were required to report the number of infected passengers and crew onboard. This should be part of their process of pratique prior to them arriving in our ports and the information should be made publicly available.

“Much of the concern about managing Covid-19 on cruise ships will now be about protecting the health of their passengers. Even prior to Covid-19, cruise ships were notorious for outbreaks of infectious disease. Much of this heightened risk is from having thousands of people spending days at a time living in a densely packed confined environment with many shared facilities where gastroenteritis and respiratory infections can spread easily. The generally older demographic of cruise ship passengers makes them more vulnerable to infection and becoming seriously ill.”

No conflict of interest.

Dr Emily Harvey, Co-lead of the Contagion Network modelling programme, COVID-19 Modelling Aotearoa, comments:

“Now that border restrictions and public health protections have been lifted in most countries around the world, we should expect that a proportion of international arrivals, be it by air or cruise ship, will be infected with COVID-19 when they disembark.

“We have consistently seen that international air arrivals, even when we required predeparture testing, had high levels of COVID infection — similar to or higher than the estimated prevalence in their country of origin. Some of this can be attributed to the increased risk of infection during travel. As an example, in mid-2022 when we had good reporting on border case numbers from compulsory RAT testing during the BA5 wave, we had the proportion reporting positive RATs get as high as 5-6% in international arrivals. This was in line with the ~6% prevalence peak seen in the UK during that period, which is the only country that conducts a regular infection prevalence survey.

“International arrivals by both air and cruise ships add increased risk of introduction of new variants, and can lead to an increase in infection risk in the community if the prevalence is higher in the country of origin than in Aotearoa.

“Although the number of people arriving on planes is far greater than by those arriving on cruise ships, there are a few key differences to consider.

“If we know the background prevalence, there is a calculation that we can do to estimate the likelihood that someone in a randomly selected group has COVID-19.

“Using this maths, we can estimate that on a plane with 300 passengers, with a background prevalence of 1%, the chance that someone on your plane is infected is 78%. Now thinking about cruise ships, which are larger, we would estimate that on a cruise ship of 2000 people, with background prevalence of 1% the probability that at least one of them is infected at the start of the trip approaches 100%.

“As context, with the current ‘lull’ in levels of COVID-19 prevalence, prevalence sits around 3% in the UK at the moment (which is the only country that conducts a regular infection prevalence survey), or around 1% in NZ (if we assume case ascertainment rates of around 35%, but we really need an infection prevalence survey here).

“Confirmed transmission on aircrafts has mostly been to a small number of people sitting nearby. However, as we have seen in past years, the longer duration and shared facilities on cruise ships has meant that a small number of infected passengers has often led to large outbreaks. Whether this continues to occur will depend on the infection control measures that are being implemented onboard the ships. I would hope that the required reporting under the Maritime Declaration of Health was being enforced and monitored by Public Health Officials, and additional measures taken if larger outbreaks were being detected.

“Rapid antigen tests are good at detecting when people are most infectious. Requiring a negative RAT result to disembark would be a simple measure that would help reduce the possibility of infectious passengers coming ashore.

“Additionally, although the number of international passengers arriving in Aotearoa on cruise ships is dwarfed by the number arriving via air travel at a national level, it is important to consider that for some regional ports in smaller towns cruise ship arrivals will add a substantial extra number of international visitors.

“Finally, the age profile of cruise ship passengers (75% being over 50, and half being over 65), adds an additional risk to consider, because these age groups are at much higher risk of needing hospital level care if infected with COVID-19. An outbreak within a cruise ship has the potential to add to the burden on our hospital system, which would again be an even greater issue in smaller regional port cities, with lower hospital capacities.”

Conflict of interest statement: “Dr Harvey’s employer receives funding from the DPMC for her to provide modelling and analysis to government officials on Aotearoa’s COVID response, and from the NZ Health Research Council for a project on modelling and equity for COVID-19 in Aotearoa.”