There is growing evidence that the prevalence of asymptomatic Covid-19 may be much higher than has been recognized, and experts say the findings have global implications for those working to ease lockdown restrictions.
Separate studies published on Wednesday, May 27, both found a high prevalence of “silent” SARS-CoV-2 infection in the populations, with more than 4-out-of-5 passengers and crew on an expedition cruise ship who tested positive in one of the studies showing no signs of illness.
The lead researcher of that study, published in the BMJ journal Thorax, had no idea when he boarded the expedition ship in mid-March that his three week trip-of-a-lifetime adventure cruise through the Antarctic Peninsula would end with more than half of the 128 passengers and 95 crew members testing positive for SARS-CoV-2.
Respiratory medicine professor Alvin J. Ing, MD, of Macquarie University in Sydney, Australia, said that’s because even though the World Health Organization (WHO) officially declared Covid-19 a global pandemic just days prior to the ship’s embarkation, every effort had been made to ensure that no infected passengers or crew ended up on the voyage.
Ing spoke about his experience and subsequent research in a podcast, noting that the cruise was planned as a three-week expedition to retrace the Antarctic route taken by British explorer Ernest Shackleton in 1915.
Passengers and crew who had been in countries with prevalent Covid-19 at the time (China, Macau, Hong Kong, Taiwan, Japan, South Korea and Iran) were not allowed on the ship, and temperatures and symptom assessments were taken prior to boarding. After boarding, everyone on the ship had their temperatures taken regularly by the ship’s two physicians.
Hand sanitizing stations were also placed throughout the ship, and all passengers were required to use them prior to dining.
“We were going to an area of the world which did not have Covid-19, so we thought we were a pretty safe bunch,” Ing said. “For the first 8 days it was virtually heaven. We enjoyed the wildlife and we enjoyed the sailing, photography, and hiking.”
On the 8th day of the cruise, Ing said, reality struck when a passenger developed a fever.
“It snowballed from there,” he added.
Isolation protocols were immediately implemented after the febrile passenger was identified, with everyone confined to cabins. Surgical masks were issued to everyone and full personal protective equipment was used for physician contact with febrile patients and N95 masks were worn by staff during contact with passengers in their cabins. Communal dining was suspended, and passengers received meals in their cabins.
Despite these precautions, three crew members were found to have fevers on day 11 and three passengers on day 12. Three more people developed fevers on day 14, including a 68-year-old male — a lifelong non-smoker with no comorbidities — who required urgent evacuation from the ship and subsequent intubation.
On day 13 of the cruise, the ship docked in Montevideo, Uruguay, after the ship’s country of origin, Argentina, had closed its borders and it was refused permission to disembark in the Falkland Islands.
RT-PCR testing of the 217 passengers and crew identified 128 who tested positive for Covid-19 (59%), including all symptomatic patients who had tested negative on a previous SARS-CoV-2 rapid test (VivaqDiag QSARS-Cov-2 IgM/IgG).
Fever and mild symptoms were recorded in just 16 of the 128 SARS-CoV-2-positive patients (12.5%), with 8 others showing more serious symptoms (6.2%) requiring evacuation. Four people (3.1%) required intubation and ventilation, and 1 patient (0.8%) died.
In 10 cases, passengers (two each) sharing the same cabin had opposite test results, indicating a high percentage of false negative findings, the researchers noted.
All passengers were allowed to leave the ship in Uruguay by day 32.
Ing said a main takeaway from the experience and the observational findings is that the prevalence of Covid-19 on cruise ships with infected people is likely to be underestimated, suggesting that monitoring all passengers after disembarkation is warranted.
In addition, rapid AB Covid-19 testing proved to be highly unreliable on the ship, and the high prevalence of testing discordant cabin-mates suggests a significant false-negative with RT-PCR testing.
The fact that some passengers developed symptoms as late as day 24 also suggests that cross contamination after cabin isolation occurred, Ing said.
In a blog post published with the study, Thorax editor-in-chief Alan Smyth, MD, noted that in early March, WHO officials suggested that only about 1% of people with Covid-19 exhibit no symptoms. The finding of an asymptomatic infection rate of 81% in this observational analysis suggests that the actual rate is much higher.
“As countries progress out of lockdown, a high proportion of infected but asymptomatic individuals may mean that a much higher percentage of the population than expected may have been infected with Covid,” Smyth wrote. “These individuals may have immunity to Covid. This demonstrates the urgent need for accurate seroprevalence studies to estimate the overall population infection rate across the globe.”
The second study, which appeared in JAMA Network Open, included 76 early Covid-19 cases from 26 cluster cases of exposure to the Wuhan, China, Hunan seafood market or close contacts with other patients.
In that case analysis, 42% of cases were found to be asymptomatic.
Asymptomatic patients tended to be younger and a higher percentage were female and they had a lower proportion of liver injuries (1 patients [3.0%] versus 9 patients [20.0%]; P=0.03), less consumption of CD4+T lymphocytes (median [IQR] CD4 lymphocyte count during recovery, 719 [538-963] per μL versus 474 [354-811] per μL; P =0.009).
They also tended to have faster lung recovery in CT scans (median [IQR] duration, 9 [6-18] days versus 15 [11-18] days; P = 0.001), shorter duration of viral shedding from nasopharynx swabs (median [IQR] duration, 8 [3-12] days versus 19 [16-24] days; P =0.001), and more stable results of SARS-CoV-2 testing (4 fluctuated results [12.1%] versus 15 fluctuated results [33.3%]).
“Our finding of less consumption of CD4+T lymphocyte in asymptomatic infections suggests that damage to the immune system in asymptomatic infections was milder compared with symptomatic infections,” wrote researcher Rongrong Yang, PhD, and colleagues.
“Although patients who were asymptomatic experienced less harm to themselves, they may have been unaware of their disease and therefore not isolated themselves or sought treatment, or they may have been overlooked by health care workers and thus unknowingly transmitted the virus to others.”
- Separate studies found a high prevalence of “silent” SARS-CoV-2 infection.
- In one of the studies, 81% of passengers and crew on an expedition cruise ship who tested positive for SARS-CoV-2 showed no signs of illness.
Salynn Boyles, Contributing Writer, BreakingMED™
Researcher Alvin Ing and colleagues declared no funding source nor relevant conflicts of interest.
The study by Yang and colleagues was funded by the Medical Science Advancement Program, Wuhan University. The researchers declared no relevant conflicts of interest.
Blog post writer and Thorax editor-in-chief Alan Smyth reported no relevant conflicts of interest.
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