The accumulated evidence suggests that most transmission is respiratory , with virus suspended either on droplets or , less commonly , on aerosols .
Kraay , et al . ( 2018 ) indicate that higher reductions on R0 can be achieved by increasing the frequency of surface decontamination . In contrast , they say , handwashing did not have an appreciable effect on R0 . They explain , “ For influenza , only higher frequency ( ≥ 1 / day ) surface decontamination strategies appear to meaningfully reducing R0 , with a maximum reduction of 40 percent in low surface-contact venues . However , fomite transmission is only possible in settings with higher touching rates and proportions of accessible surfaces . Thus , surface decontamination for influenza may prevent outbreaks in venues with moderate surface contact rates and many accessible surfaces . In contrast , in our simulations similar interventions for rhinovirus and norovirus were not effective , even with cleaning frequencies of up to twice per day . For rhinovirus , an effect of no more than 5 percent is observed and for norovirus the maximum effect size was 20 percent . Even with this 20 percent effect size , our model suggests that R0 would remain substantially above 1 . While higher cleaning frequencies may demonstrate improved efficacy , they are likely not practical to implement and so were not considered in this analysis .”
From their findings , the researchers emphasize , “ Fomites are an important source reservoir for pathogens that can persist in the environment . Environmentally persistent pathogens like norovirus and rhinovirus are able to exploit fomite pathways in a variety of potential venues . For pathogens with higher surface die-off rates such as influenza , fomite transmission is sustainable in a narrower range of venues . Based on these findings , the fomite-mediated pathway may be sufficient to sustain transmission even if interventions targeting more direct pathways are successful . Focusing on fomite-mediated routes allow us to examine a transmission pathway that might otherwise be masked by faster processes such as direct transmission and provides a natural way to evaluate the effectiveness of environmental interventions such as surface decontamination . While we do not consider transmission that might occur through other routes , we show that the extent to which environmental interventions can successfully control fomite-mediated transmission is affected by both the venue in which transmission occurs ( both physical properties and behavioral factors ) and intrinsic pathogen properties ( including inactivation rates , transfer efficiencies , and shedding rate ).”
As we have seen , there is debate in the literature regarding SARS-CoV-2 contamination as being primarily airborne and not fomite-driven .
Meyerowitz , et al . ( 2020 ) reviewed the evidence on transmission of SARS-CoV-2 , noting that , “ Although several experimental studies have cultured live virus from aerosols and surfaces hours after inoculation , the real-world studies that detect viral RNA in the environment report very low levels , and few have isolated viable virus . Strong evidence from case and cluster reports indicates that respiratory transmission
• is dominant , with proximity and ventilation being key determinants of transmission risk . In the few cases where direct contact or fomite transmission is presumed , respiratory transmission has not been completely excluded .”
In their review , the researchers found that respiratory transmission is the dominant mode of transmission and that vertical transmission occurs rarely ; they also identified that direct contact and fomite transmission are presumed but are likely only an unusual mode of transmission . At the onset of their review , they sought to answer the questions :
● what is the evidence for the environmental viability of the virus in experimental and real-world settings ?
● what viral and host factors affect transmission ?
● what is the evidence for various modes of transmission ?
● what is the period of infectiousness for a person with SARS-CoV-2 infection ?
● what are the population transmission dynamics , and what is the role of superspreading events ?
Within experimental conditions , Meyerowitz , et al . ( 2020 ) cultured viable SARS-CoV-2 from aerosols and various surfaces after inoculation with 105.25 50 percent tissue culture infectious dose per milliliter ( TCID50 / mL ) for aerosols and 105 TCID50 / mL for surfaces , correlating to a reverse transcriptase polymerase chain reaction cycle threshold of 22 to 24 , a typical value obtained from a nasopharyngeal sample of a person with COVID-19 . They found that viral RNA decayed steadily over time in all conditions , although viable virus was isolated for up to three hours from aerosols and up to 72 hours from various surfaces ; the longest reported viability was on plastics and stainless steel , with half-lives around six hours . As they note , “ In real-world settings , studies have identified SARS-CoV-2 RNA from samples taken from contaminated environmental surfaces , most commonly high-touch surfaces . Viral RNA levels are markedly lower on environmental surfaces than in the nasopharynx of source individuals , as shown in studies . The few studies that have assessed the presence of replication-competent virus with culture have isolated it rarely in air particles of varying size .”
Regarding direct contact and fomites , Meyerowitz , et al . ( 2020 ) say , “ There is currently no conclusive evidence for fomite or direct contact transmission of SARS-CoV-2 in humans . Reports suggesting fomite transmission are circumstantial .” For example , in a detailed investigation of a large nosocomial outbreak linked to 119 confirmed cases at a hospital in South Africa , fomite transmission was proposed given the separated distribution of cases in multiple wards , however , the hospital did not have a universal mask policy at the time of the outbreak , there was no special ventilation , and the burden of infection among healthcare workers was substantial . As a result , respiratory transmission from infected staff cannot be excluded .”