“Kills 99 percent of all known germs”, “kills 99.9 percent of all germs and viruses!” – we’ve got used to such advertisements for hand sanitisers these days, haven’t we? But where is the evidence behind such claims? And what exactly do they mean?
Is that 99 percent microbe mortality rate a population proportion? Are we leaving 1 percent of the germ population there ready to replicate and take advantage of the vacuum effect we know allows badgers to repopulate an area when we’ve slaughtered most of them in an attempt to reduce bovine TB? Viruses can have replication rates in tissue culture of up to a thousand billion a day. So, it won’t take long – a few minutes – for that remaining 1 percent to reach the original level, will it? And the fact that a 99.9 percent effective hand sanitiser kills 10 times as many germs as one that merely kills 99 percent is hardly here or there!
Or maybe it kills 99 percent of microbial species? In that case we have to hope that the 1 percent remaining aren’t the most virulent, don’t we? And what are those “germs”? Bacteria? Viruses? For all their potential nastiness, bacteria are fairly fragile organisms in the face of detergent or alcohol. But how do you kill a virus that’s just protein and nucleic acid? Maybe we should say inactivate rather than kill? Enveloped viruses like COVID-19 and influenza are readily inactivated by alcohol or detergent. But what about non-enveloped viruses like norovirus in people or, for companion animal species, similar viruses like feline calicivirus? Actually, alcohol works for those viruses too, but interestingly not at high concentrations such as 90 percent. The most efficacious antiviral solutions have lower percentages of alcohol. The trouble is high concentrations are best for COVID-19. So perhaps “all germs” turns into “most germs”?
Then there’s the addition of “known” into the equation. That does seem to be reasonable. If there’s a microbe out there that isn’t yet known, it would be difficult to ensure that we’re going to be able to kill it, wouldn’t it? But who would believe that with the expertise of microbiologists these days there might be out there an unknown pathogen? I think we know all too well now that unknown pathogens may be a lot more pathogenic than the known ones. Maybe the limitations of science are something that 2020 has taught us.
So where is the evidence on the virucidal efficacy of hand sanitisers?
The glory of Google Scholar and PubMed is that we can rapidly scan the whole of scientific literature to find out. Buckle up for a rollercoaster ride through acronyms! The CEN (European Committee for Standardisation) and the FDA (Food and Drug Administration) together with the ASTM (American Society for Testing and Materials) have come up with a plethora of standards while the DVV (German Society for the Control of Virus Diseases) and the RKI (Robert Kock Institute) have their own set.
A quick meta-analysis (Singh et al., 2020a) suggests that 85% ethanol gel, and 45% isopropanol plus 30% n-propanol both have a better than 5 log(10) reduction factor – ie they are pretty good! How though have these figures been arrived at? In vitro and laboratory tests are all well and good, but how does that fit with what happens in reality? It’s all a mix of the delivery system (gel, foam, rinse or wipe), the volume and contact time together with any contamination (oil, dirt or grease), quite as much as the type of alcohol and the concentration, to give a significant effect. The bottom line is that isopropanol is more effective than ethanol because it has a great lipophilicity but even so at least 3ml should be used for 45 seconds or more.
Ah – so perhaps our actions in using these sanitisers are quite as important as their exact formulation. We know we have to use them, or wash our hands, for as long as it takes to sing Happy Birthday twice, but do we really do that?
In another paper (Singh et al., 2020b), it is suggested that the widespread employment of these alcohol-based sanitisers has lowered the use of simple hand washing which is actually more effective. The more you read the more confused you get!
Where does this leave us then? Maybe “Gives a greater than 5 log(10) reduction of enveloped viruses when used in adequate volume and for sufficient time” should be the message on those bottles? But somehow, I guess we’ll stay with “Kills 99 percent of all known germs”, whatever the evidence is!