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Are you puzzled about how long you need to keep your food packages undisturbed to inactivate the virus that causes COVID-19? Then this tweet thread is for you. (1/34)
First let me start by suggesting that there& #39;s no evidence of transmission of COVID-19 from food or food packaging. There& #39;s also no reason to disinfect food packaging after arriving home from the grocery store. (2/34)
But let& #39;s say there is some theoretical risk there that we are trying to assess or manage. What would we need to know? How would we assess then manage that risk? (3/34)
We should start with the idea that virus inactivation is dependent on temperature. Inactivation happens faster at higher temperatures and slower at lower temperatures. (4/34)
I took data from a couple of different papers to try to figure out the effect of temperature on the rate of reduction of the virus. There& #39;s a paper on the original SARS virus: https://pubmed.ncbi.nlm.nih.gov/15350737/ ">https://pubmed.ncbi.nlm.nih.gov/15350737/... (5/34)
And then there& #39;s this pre-print on the new SARS-CoV-2: https://www.medrxiv.org/content/10.1101/2020.03.15.20036673v2.">https://www.medrxiv.org/content/1... And the data do lineup pretty well. (6/34)
From the data in these two papers I was able to construct a table which shows the relationship between temperature and rate of reduction. It will be a side tweet off of this tweet in the thread. (7/34)
There& #39;s also some other data that I& #39;ve gotten a lot of attention. These come from the NEJM https://www.nejm.org/doi/full/10.1056/NEJMc2004973">https://www.nejm.org/doi/full/... those authors report a worst case half-life of 6.8 hours on plastic. Let& #39;s round up to seven hours for simplicity. (8/34)
I should also note that the NEJM paper does not mention experiment temperature, so let& #39;s assume room temperature (~72 F). These data don& #39;t line up exactly with the earlier data, but they& #39;re in the right ballpark. (9/34)
What does it mean to have a "half life"? It means that the viable virus concentration drops by 50% every seven hours. Food microbiologists reading this may see that it& #39;s possible to convert "half-life" to "log reduction rate", that we commonly use. (10/34)
In this case a half-life of seven hours corresponds to roughly a one log reduction per day (actually about 23.26 hours, but you get the idea). That& #39;s actually slightly faster then the rate I calculated in the table above. (11/34)
Next, let me suggest that you stop thinking about the amount of "time" needed to make something safe. It& #39;s not about *time* it& #39;s about rate. What do I mean by that? (12/34)
"Inactivation time" is a function of the starting concentration, the rate of reduction, and the growth sensitivity of your method (i.e. detection limit), so any mention of "time" presupposes those three values. (13/34)
What would be a logical starting concentration? We know based on research with and without face masks: https://www.nature.com/articles/s41591-020-0843-2">https://www.nature.com/articles/... that 30 minutes of breathing without a mask gives about 10,000 virus particles from some individuals. (14/34)
Those virus particles spread throughout a store are mostly going to end up on the floor, but even if they end up on food they will gradually inactivate overtime and as you& #39;ll see below transfer rate to hands would be low. (15/34)
From the same paper swab samples of nose and throat (unknown volume) give ~1M virus particles, which might be a worst case sneeze or cough. But what does any of this mean in terms of risk? (16/34)
It means based on the NEJM letter that your chance of finding an infectious virus particle someone has sneezed, coughed or breathed on food drops by 50% every seven hours, or by one order of magnitude (1 logarithm) every day. (17/34)
But how do we know if we are going to get sick? What& #39;s the risk of illness from one or more virus particles? (18/34)
To answer this question we need a dose response model. We don& #39;t have a dose response model SARS-CoV-2, but we do have one that& #39;s been estimated for the original SARS. https://onlinelibrary.wiley.com/doi/full/10.1111/j.1539-6924.2010.01427.x?casa_token=YIN_ENJl1IUAAAAA%3ABPeAeBsZxt92sO9junbqMLdPNi-eEK22siDSc5tF_OJMI59zAv-qGAEH75V_ZBjCES2Pgn4aGOIrKQ">https://onlinelibrary.wiley.com/doi/full/... (19/34)
Based on that model if you ingest 10,000 virus particles the odds of getting sick are 100%. If you ingest 284 particles your odds are about 50-50, and if you ingest a single viral particle your risk is just a fraction of 1% (0.24% to be exact). (20/34)
But what happens when the predicted number of virus particles on a surface is less than one? Does it mean there& #39;s no virus particles? No. (21/34)
If we predict that the number of virus particles is 0.1, another way of thinking about that is if we tested 10 surfaces, we would expect to find a single virus particle on one of them. (22/34)
I think it& #39;s pretty unlikely that those hypothetical virus particles sitting there on packages of food are going to suddenly jump off and aerosolize, but they could get on your fingers. (23/34)
We don& #39;t have data on cross-contamination for the virus that causes COVID-19, but based on our research on cross contamination with other organisms, I& #39;d estimate about 1% of the virus particles (plus or minus) will transfer to the hand. (24/34)
Of course at this point it would be a really good time to wash your hands or use hand sanitizer. Probably not a good idea to stick your finger in your nose. (25/34)
So where does this leave us? If you think about the math, what it means is that nothing is ever completely safe. Sorry if that freaks you out a little bit, but that& #39;s the nature of risk and probabilities. (26/34)
So what to do? Well, start by not leaving your groceries on your porch... if it& #39;s cold outside, it won& #39;t help. I& #39;m still not planning on disinfecting my groceries either, because the risk is very low. (27/34)
The real risk is other people coughing/sneezing/breathing one *you* in the grocery store. Now that CDC is recommending face covers (i.e. masks) https://www.cdc.gov/coronavirus/2019-ncov/prevent-getting-sick/cloth-face-cover.html">https://www.cdc.gov/coronavir... that risk should be less too. (28/34)
I will keep washing my hands and using hand sanitizer, especially when returning from the store or any other trip. (29/34)
If it makes you feel better to sanitize your groceries (or for that matter wash your produce in soap), go for it, but I don& #39;t want to know. (30/34)
Thanks as always for reading. If you have questions or comments, I& #39;ll do my best to read and respond. (31/34)
Trolls will be blocked. People that just want to argue will eventually be muted. (32/34)
If you think I should& #39;ve made a video instead, sorry. You& #39;re free to make your own video and use this as your script. (33/34)
Stay home if you can, wash your hands and use hand sanitizer, and take care of those who need it most.ļæ¼ļæ¼ (34/34)
