Air Recirculation Role in the Spread of COVID-19 Onboard the Diamond Princess Cruise Ship during a Quarantine Period

The Diamond Princess cruise ship is a unique COVID-19 transmission case because of the high testing capacity and the confined environment. This exploratory study aims to raise the hypothesis regarding the role of poor ventilation systems in the spread of COVID-19 by analysing count data collected by the onboard clinic during the outbreak, and considering the deck plan and design of the air conditioning system of the ship. Observed symptomatic infection rate after day 5 (incubation period median day) of the quarantine, in cabins without previous confirmed cases are compared to that in cabins with previous confirmed cases. Accordingly, the observed symptomatic infection rate in cabins without a previously confirmed case (1.2%) was higher than for cabins with a previously confirmed case (0.8%); however, the difference was not statistically significant. In addition, age did not appear to be a confounding variable. Airborne transmission of COVID-19 through the ventilation system onboard could explain the higher than expected virus spread into cabins without previously confirmed cases during the quarantine period; thus, this study provides further potential evidence of coronavirus transmission by aerosols. Conflicting results from other studies involving the Diamond Princess outbreak are also discussed in light of our results.

Where QPM is the quarantine period after the incubation median day (day 5 of the quarantine 118 period), and CCs are the confirmed cases. After calculating in cabins with previous 119 confirmed cases and in cabins without previous confirmed cases (Table S1, and Table S2). Two-120 proportions z-test (one-tailed) was used to compare these infection rates, in which the null 121 hypothesis was: 122 Where is the infection rate in cabins without previous confirmed cases after the 125 incubation period median day. And is the infection rate in cabins with previous 126 confirmed cases after the incubation period median day. 127 Any age group that was found to be less than 5 people was merged with the closest age 128 group to it. Chi-square test was used to check whether symptomatic, asymptomatic, non-case 129 counts differ by ages. If a significant difference was found, relative contribution of each cell to the 130 total chi-square score were used to check where the differences actually lie. The significance 131 level for all tests is 0.05. R (version 3.6.1) was used to run the statistical analysis. 132 Literature review: The Diamond Princess's HVAC filtration system design was examined 133 by checking the Diamond Princess's website (Princess, 2020) and referencing Kosako and 134 Shiiyama (2008) that extensively described the air conditioning system design for large cruise ships 135

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6 of the Princess Cruise Mitsubishi Grand implementation. This Grand series consists of two similar 136 cruise ships; Diamond Princess and its sister Sapphire Princes. Moreover, the deck plan of 137 Diamond princess cruise ship was checked to assess the percentage of cabins that depends totally 138 on the supplied air by the ship's HVAC system (Cruise Deck Plan, 2020). 139 As this study was secondary retrospective analysis of anonymised published count data, 140 formal Research Ethics approval was not required. 141

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The number of new symptomatic confirmed cases in all cabins continued to decline as the 145 quarantine continued (Fig. 1). This may have been due to the strict infection prevention and control 146 measures that were put in place during the quarantine period onboard the ship (Zhang et al., 2020;147 Mizumoto and Chowell, 2020) resulting in a substantial decline of the dominant transmission mode 148 onboard, i.e. passenger-to-passenger transmission, as a result of passengers staying inside their 149 cabins . Alternatively, a large proportion of asymptomatic cases 150 may have remained undetected (Emery et al., 2020; Plucinski et al., 2002). 151 Based on the assumptions that the exposure to external COVID-19 transmission routes 152 during the quarantine period was the same for all passengers' cabins, and the infection risk was 153 higher when there was a close contact with a sick person, a lower infection rate after the incubation 154 period median day was expected in cabins without previous confirmed cases than that in cabins 155 with previous confirmed cases. Remarkably, SIRR in cabins without previous cases (1.2%) 156 was found higher than that in cabins with previous cases (0.8%) after the incubation median day. 157 However, there was not sufficient evidence to reject the null hypothesis at 0.05 level of significance, 158 in which difference in was 0.4% (p value=0.2, 95% lower confidence interval bound: -159 0.3%). This, in part, may have been due to the small numbers of cases with recorded symptom 160 onset dates after day 5 of the quarantine resulting in the test being underpowered to detect a 161 significant difference. In addition, the day-to-day contribution of asymptomatic cases to 162 symptomatic infection rates in cabins during the quarantine period was not known. Other studies 163 (Plucinski et al., 2020;Emery et al., 2020; have estimated that the 164 proportion of asymptomatic infections could range from 17.9 to 74%. The asymptomatic 165 proportion was defined as the proportion of asymptomatically infected individuals among the total 166 number of infected individuals . 167 Analysis of the second prepared dataset (Table 2) where children and young people under 168 20 years of age were excluded due to small sample sizes, 80-89 and 90-99 age groups were 169 combined, and the sums of confirmed case were divided by the population at risk per age group, 170 showed that the highest infection rate was in the age group of "80-99" (23%) (Fig. 2). Chi-squared 171 test showed that case distributions differed significantly by age group (df=12, P <0.0001). However, 172 asymptomatic confirmed cases in the age groups: "70-79", "20-29", "30-39", "40-49" (in 173 descending order of contribution) contributed approximately 71% of the total chi-square score and 174 thus account for most of the difference. Symptomatic confirmed cases contributed to only 10% of 175 the difference. Accordingly, a hypothetical difference in age distributions among the cabins cannot 176 explain why the symptomatic infection rate in cabins with previous confirmed cases was not higher 177 than that in cabins without previous cases. Plucinski et al. (2020) verified the association between 178 age and presenting with symptoms. 179 In their paper about the central HVAC system design in the Diamond Princess cruise ship, 180 Kosako and Shiiyama (2008) stated that although cruise ships should be able to use 100% fresh air 181 as a design condition; it is a part of energy-saving measures to mix outside air with that inside. 182 Cabin temperature regulation happens through a combination of reheating and variable airflow 183 (Kosako and Shiiyama, 2008). It is ordinary practice to use only 30% of fresh air inside passengers' 184

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8 cabins (Kosako and Shiiyama, 2008). As a result, air from different cabins can gather in one air 185 duct, mixed with fresh air, be filtered then re-circulated to cabins. 186 Diamond Princess' deck plans showed that about 30% of the cabins onboard are interior 187 cabins and with no access to the natural fresh air (Cruise Deck Plans, 2020). Furthermore, given 188 the COVID-19 outbreak on the Diamond Princess cruise happened during winter, meant even 189 passengers in cabins with balconies were dependent on the HVAC system. On their website 190 (Princess, 2020), and when they were asked about whether COVID-19 can be spread through the 191 ship's ventilation system, Princess did not declare, in their answer, that they had stopped the HVAC 192 during the quarantine period, nor they had operated the HVAC system on 100% fresh air supply. 193 They only had confirmed that the HVAC filtration system on the Diamond Princess ship is 194 comparable to those used by land-based hotels, resorts and casinos (Princess, 2020). 195 HVAC systems in commercial setting have a minimum efficiency reporting value (MERV) 196 of 5-8 (Table S3), in which MERV refer to the effectiveness of air filters in HVAC. And even in 197 superior residential, commercial, and industrial spaces HVAC systems usually have a minimum 198 efficiency reporting value (MERV) of 9-12 (Table S3) Similarly, one preprint study ) that predicted the dates of infections on the 220 Diamond Princess cruise ship reached an opposite result to ours, in which they, using their 221 predicted data concluded that "the ship central air conditioning system did not play a role in the 222 transmission" and "the long-range airborne route was almost totally absent in the outbreak". The 223 differences in our studies are as follows; firstly, they have claimed that infection among passengers 224 after 6 th of Feb was limited to those who stayed in the same cabin with an infected passenger. 225 Though the published data (NIID, 2020), clearly stated that among the confirmed COVID-19 cases 226 with recorded symptom onset, 92 cases occurred among passengers in cabins without a previously 227 confirmed cases during the quarantine period. Moreover, in the period after the incubation median 228 day during the quarantine, 71% of the new cases occurred in cabins without a previous confirmed 229 case, and 29% occurred in cabins with a previous confirmed case. And secondly, Xu et al., (2020) 230 claimed that "It was also reported that the maximum outdoor air supply was operated during the 231 quarantine period," but did not provide a reference to support this claim, and still to date, we could 232 find no evidence that the Diamond Princess cruise ship had actually operated its HVAC on a 233 maximum air supply during COVID-19 outbreak. Finally, the authors reported that "During the 234 quarantine period, passengers stayed in their rooms, and the outer cabins had access to balcony 235 doors" and they did not reveal that at least 30% of the cabins onboard, and which are usually the 236 most occupied cabins due to their lower price, had no access to fresh air and were totally dependent 237 on the ship' HVAC because they are interior cabins. 238 Nonetheless, and close to our results, a study that analysed two COVID-19 outbreaks on 239 buses and places of worship has strongly suggested that the conditioning system on a re-circulating 240 mode may have facilitated the spread of COVID-19 virus during outbreaks (Shen, 2020). The same 241 study showed that passengers sitting closer to the index case on the exposed bus did not have 242 statistically higher risks of COVID-19 as those sitting further away. The study also showed that 243 all passengers sitting close to a window remained healthy which may be due to better airflow (Shen, 244 2020). viruses are carried by dust or droplet nuclei suspended in air (Remington et al., 1985). Airborne 265 dust can be resuspended by air currents after settling on any surface and droplet nuclei could remain 266 suspended in the air for a long time and may travel long distances (Remington et al., 1985). and recommend that adequate control measures should be implemented to prevent further spread 305 of the COVID-19. The study (Morawska and Cao, 2020) quoted another study (Qian and Zheng,306 2018) when mentioning all the possible precautions that should be taken against airborne 307 transmission in indoor settings. These precautions include increased ventilation rate, using natural 308 ventilation, avoiding air recirculation, avoiding staying in another person's direct air flow, and 309 minimizing the number of people sharing the same environment (Qian and Zheng, 2018). 310 Our current study has several limitations. First and foremost, the incompleteness and quality 311 of the published data used to compare the infection rates. Only summary data were available, and 312 some counts were given as approximations. Data were not available on relevant factors such as the 313 spatial distribution of passengers in the interior and outer cabins, and passengers' prior health 314 conditions. Second, assumptions such as interaction levels with the crew members were the same 315 for all passengers regardless whether their cabins had or had not previous confirmed cases may not 316 be accurate. Third, though we are not discounting other important transmission modes such as 317 close-contact droplets and fomites, in this study, we only considered the airborne transmission 318 mode to explain why the infection rate with COVID-19 in passengers' cabins with previous cases 319 was not higher than in cabins without previous confirmed cases after day 5 (incubation period 320 median day) of the quarantine. Finally, we have not conducted an experiment to test the proposed 321 airborne transmission mode through the HVAC system. 322

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Airborne transmission of COVID-19 through poorly filtered and poorly ventilated, 326 recirculated air onboard ship could explain the higher than expected virus spread into cabins 327 without previously confirmed cases during the quarantine period. 328 There is accumulating evidence of COVID-19 spreading widely in confined settings such 329 as restaurants, hospitals, care homes, shops, gyms, public transport, offices, schools, prisons, etc. 330 Ventilation system design, filters, and upgrades; natural ventilation (just using outside air and not 331 recirculating it); and airflow (direction/speed) should be all considered and evaluated when 332 deciding what intervention measure(s) is appropriate to reduce exposure and limit the transmission 333 of COVID-19 in a confined setting. Keeping two meters distance between customers in a shop, for 334 instance, is not an effective measure without considering the air flow inside the shop. Self-isolating 335 residents of a care home inside the rooms is not an effective measure if the ventilation system in 336 the care home does not have highly-efficient filters that can capture a virus as small as COVID-19, 337 especially if the HVAC system is working on a save-energy mode and the air is kept recirculated 338 inside the residence.