According to research published Friday by UC San Diego School of Medicine and San Diego State University researchers, the risk of contracting COVID-19 from handling trick-or-treat candy that has been in contact with a coronavirus-positive person is minimal, but not zero.
In the study published Friday in the journal mSystems, the researchers analyzed the viral load on Halloween candy handled by patients with COVID-19.
SARS-CoV-2, the virus which causes the illness COVID-19, is primarily transmitted by respiratory droplets and aerosols. The risk of infection by touching fomites — objects or surfaces upon which viral particles have landed and persist — is relatively low, according to multiple studies, even when fomites are known to have been exposed to the novel coronavirus. Nonetheless, the risk is not zero.
"The main takeaway is that although the risk of transmission of SARS- CoV-2 by surfaces — including candy wrappers — is low, it can be reduced even further by washing your hands with soap before handling the candy and washing the candy with household dishwashing detergent afterwards," said co-senior author Rob Knight, professor and director of the Center for Microbiome Innovation at UCSD.
"The main risk is interacting with people without masks, so if you are sharing candy, be safe by putting it in dish where you can wave from six feet away," he said.
As San Diego County heads into a Halloweekend, public health officials are urging members of the public to practice COVID-19 protocols — including avoiding large gatherings such as Halloween parties and door-to-door trick-or-treating.
"These activities involve face-to-face interactions with people from different households," said Dr. Wilma Wooten, the county's public health officer. "If a COVID-19 infection is detected among a participant, it will be very difficult to find and notify those who may have been exposed."
These traditional Halloween celebrations are not advised, and large gatherings are not allowed under state or local health guidance. The county has reported dozens of community outbreaks in the past week.
For their study, the researchers enrolled 10 recently diagnosed COVID- 19 patients who were asymptomatic or mildly symptomatic and asked them to handle Halloween candy under three different conditions: Normally with unwashed hands, normally with washed hands and extensive handling while deliberately coughing.
The candy was then divided into two treatments — no post-handling washing and washed with household dishwashing detergent — followed by analyses using real-time reverse transcription polymerase chain reaction, the same technology used to diagnose COVID-19 infections in people, and a second analytical platform that can conduct tests on larger samples more quickly and cheaply. Both produced similar findings.
On candies not washed post-handling, researchers detected SARS-CoV-2 on 60% of the samples that had been deliberately coughed on and on 60% of the samples handled normally with unwashed hands. However, the virus was detected on only 10% of the candies handled after handwashing.
The dishwashing detergent was effective for reducing the viral RNA on candies, with reducing the viral load by 62.1 percent.
The researchers had also planned to test bleach, but noted that bleach sometimes leaked through some of the candy wrappers, making it unsafe for this type of cleaning use.
The study authors underscored that the likely risk of SARS-CoV-2 transmission from candy is low, even if handled by someone with a COVID-19 infection, but it can be reduced to near-zero if the candy is handled only by people who have first washed their hands and if it is washed with household dishwashing detergent for approximately a minute after collection.
Knight led the study with Forest Rohwer, viral ecologist at San Diego State University, and Dr. Louise Laurent, professor at UC San Diego School of Medicine.
