Covid-19 patients most infectious in 1st week of illness, but odds of transmission significantly reduced after 2nd week: S’pore study

SINGAPORE — The odds of Covid-19 patients infecting others through the air and contaminated surfaces peak in the first week of illness. 

But after the second week, the possibility of infecting others becomes negligible, said Singapore researchers who tested air and surface samples of hospital rooms where Covid-19 patients were treated. 

These were the findings from a recent study by researchers from the National Centre for Infectious Diseases (NCID), Duke-NUS Medical School and DSO National Laboratories. 

The results were published in the Nature Communications scientific journal late last month. 

This follows earlier research from Singapore in March, which showed that coronavirus patients widely contaminate their bedrooms and bathrooms.

The latest study — the findings of which were announced in a press statement released on Friday (June 19) — is only the second in the world to examine particle-size distribution of Sars-CoV-2, the virus that causes Covid-19, in the air surrounding patients. 

Researchers tested surface samples in 30 negative-pressure hospital rooms that are designed to isolate airborne infectious diseases. 

More than 200 surface samples were taken from the rooms — three in the intensive care unit (ICU) and 27 in the general ward.

Air samples were taken from three of the general wards, where Covid-19 patients in various stages of illness were treated. To test air contamination, researchers collected particles using bio-aerosol samplers at different heights — 1.2m, 0.9m and 0.7m — near the air exhaust in the wards for four hours. 

To test surface contamination, samples were collected from surfaces that were frequently touched, including bed rails, bedside lockers, bed control panels and call bells.

Also sampled were toilet seats and automatic flush buttons in the general ward rooms.

FINDINGS

The study found that the presence of contamination on high-touch surfaces was significantly higher in the rooms of patients during their first week of illness. 

The extent of contamination declined over time. 

Although seven of the 30 patients were asymptomatic during the sampling, the study showed that patients could shed high virus loads on surfaces, irrespective of their symptoms. 

The floor was most likely to be contaminated (65 per cent), followed by the air exhaust vent (60 per cent), bed rail (59 per cent) and bedside locker (47 per cent). 

The toilet seats and flush buttons of rooms housing five patients were found to be contaminated. All five occupants had reported gastrointestinal symptoms within a week of sampling. 

No surface contamination was detected in the three ICU rooms, indicating that patients in intensive care shed less virus as they are usually in the second week of illness, the researchers said. 

The three rooms in the general wards that were tested for air contamination also showed varying results.

Sars-CoV-2 particles, between 1 and 4 micrometres (or microns) in diameter, were reported in two rooms. Both patients were on the fifth day of illness with high viral loads derived from nasopharyngeal swabs.

There was no sign of the coronavirus in the air samples of the third room. The patient was on the ninth day of illness, with lower nasopharyngeal viral loads.

Dr Kalisvar Marimuthu, a senior consultant at NCID and the study’s principal investigator, said these findings supported other studies, which showed that Covid-19 patients are not infectious after Day 11 of illness.

The findings, he said, also revealed that the virus is present in a particle size small enough to suggest aerosolising potential. But this alone is insufficient to prove that the coronavirus is airborne, as “the viability of the virus in the air will need to be proven”, he added.

Professor Leo Yee Sin, NCID’s executive director, said that current epidemiological findings of coronavirus transmission also do not point to airborne transmission on the scale of measles and varicella, the virus that causes chickenpox.

Dr Kristen Coleman, a research fellow with the emerging infectious diseases programme at Duke-NUS Medical School, said the next step is to determine the proportion of expelled viruses that is infectious. 

“This is what the world is waiting to know,” she said.

NCID, Duke-NUS Medical School and DSO National Laboratories are investigating the distance and duration over which the virus is able to travel in the air to find out its airborne potential.

The researchers said the findings from their study show not only patterns of air and environmental contamination, which are essential for the improvement of medical standards. 

They also help authorities to debunk public misconceptions about the infective nature of the virus and “minimise social discrimination of discharged patients”.