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People who are infected with COVID-19 but show no symptoms could make up a significant proportion of COVID cases and have an important impact on the virus’s spread, suggests a new by the London School of Hygiene & Tropical Medicine.

The research team estimated that 72% people of infected with COVID-19 on board the Diamond Princess cruise ship remained asymptomatic. For each asymptomatic case, it remained uncertain how infectious they were although they were unlikely to be as infectious as a symptomatic case.

Overall, the team estimate that asymptomatic cases were responsible for 11-84% of all infections during the outbreak. Work will be extended with more data (where available) and to other outbreaks, which the team hopes will enable more accurate estimates of the impact of asymptomatic cases on all infections over the coming period.

Rein Houben from LSHTM’s Centre for the Mathematical Modelling of Infectious Diseases who led the research, said: “We know that a substantial number of people infected with the coronavirus are asymptomatic, and there is anecdotal evidence that this group can transmit the virus, However, there are few reliable estimates on the proportion of people with COVID19 who don’t show symptoms, and their overall contribution to the spread of the virus.

“This information is crucial for health policymakers to develop the most effective response strategies, including exiting from lockdowns. Using data from the Diamond Princess cruise ship, we aimed to help fill this knowledge gap.”

On 3 February 2020, an outbreak of COVID-19 was reported on the Diamond Princess cruise ship off the Japanese coast. Over the next month, some 700 people had tested positive and sadly nine died.

Over 80% of passengers and staff were tested at least once for infection, and dates of testing and dates of developing symptoms for each person was recorded. By carefully matching this reported data with a mathematical model, the research team uncovered the spread of the virus amongst passengers and crew.

They then followed how this changed with isolation of symptomatic cases, quarantine of passengers and finally mass-testing, regardless of symptoms. This allowed them to estimate how many cases were asymptomatic, and how they contributed to transmission.

The model estimates that a third of people on board were infected by the end of the outbreak, but half of all infections went undetected as testing regardless of symptoms only started towards the end of the outbreak.

In the absence of testing asymptomatic individuals, a third of cases, almost all asymptomatic, had completed their infectious period and recovered before they were tested.

Rein Houben said: “While asymptomatic cases were likely less infectious than compared to clinical cases, they may well have contributed significantly contributed to overall transmission. As the proportion of people with asymptomatic disease may be higher in younger populations than in this outbreak. Control measures, and models projecting their potential impact, need to look beyond the clinical cases if they are to understand and address ongoing transmission.

“Extensive testing of non-symptomatic individuals will most likely need to be part of any effective control or exit strategy.”

The authors acknowledge limitations of their work, including the remaining limitations in the data, and the inability of the model to identify a range for the relative infectiousness of asymptomatic infections.

***This study has not been peer reviewed***

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