There is no convincing evidence that nasal sprays prevent COVID-19

https://mathissweet.substack.com/p/there-is-no-convincing-evidence-that

“There is a lot of misinformation out there about nasal sprays preventing COVID-19. Unfortunately, there are no convincing studies showing that nasal sprays prevent COVID-19. The published studies investigating whether or not nasal sprays prevent COVID-19 each have major issues, which I will detail here.
I have a PhD in biochemistry and one of my PhD projects was on COVID-19. The main takeaway of this post is that there is no sound evidence that nasal sprays prevent COVID-19. Thus, nasal sprays should not be used for COVID-19 prevention in place of effective measures such as high-quality well-fitting respirators, ventilation and air purification.
This post has become long, so here are the sections in order as they appear:
1. Brief overview of issues with the studies
2. Human clinical trials with placebos
3. Studies in humans without placebos (which are not clinical trials)
4. Studies in test tubes/cell culture and why that isn’t transferable to the human respiratory tract
5. Summary/TLDR and final thoughts
I will name the COVID-19 prevention nasal spray studies I’m going over study 1, study 2, etc. and for other papers cited I’m naming them study A, study B, etc. Basically, I want to make sections of this post easy to refer to and discuss. And if there are other human clinical trials looking at nasal sprays for preventing COVID-19 let me know and I will review them and edit the post to add them in.
1. As a brief overview, some major issues with these studies include:
• The fact that the test spray and not the placebo spray contain ingredients that can cause false-negative COVID-19 tests (combined with no information on the timing between applying nasal sprays and taking nasal/nasopharyngeal swabs for COVID-19 tests)
o Ex: a heparin nasal spray can cause false-negative COVID-19 RT-PCR tests (study A) and carrageenan from vaginal swabs after using carrageenan-containing lube can cause false-negative PCR tests for HPV (study B). If we take the estimate from another paper (study C) that nasal sprays get immediately diluted approximately 1:1 by nasal fluid (when the spray volume in each nostril is 0.100 mL), then the amount of carrageenan in a nasal swab taken immediately after spraying the nasal spray is comparable to that in the carrageenan undiluted samples in experiment 4 in study B. Those samples from study B all produced false-negative PCR tests for HPV. (EDIT APRIL 13, 2025: study R shows carrageenan nasal sprays causing false-negative COVID-19 RT-PCR test results and reductions in measured viral loads.)
• Lack of placebo spray, participants having to seek out the test spray themselves (suggesting they may take more precautions than those in the study taking no spray, not even a placebo)
• Lack of sufficient information for reproducibility (especially regarding what is considered a positive and a negative COVID-19 RT-PCR test result)
• Lack of testing for asymptomatic/presymptomatic infections (how can we say something prevents COVID-19 if we aren’t testing for asymptomatic and presymptomatic COVID-19 infections?)
• Inappropriate COVID-19 testing methods
• Wide 95 % confidence intervals for relative risk reductions
• The group promised a follow-up study with more participants and the trial was completed but the results were never posted (suggesting that the results did not show the test spray preventing COVID-19)
o Ex: in study D a protocol was published for an upcoming carrageenan nasal spray clinical trial, and that trial finished in 2022 but the results haven’t been posted. Generally, if you do a search on clinicaltrials.gov with the condition “COVID” and the intervention/treatment “nasal spray”, you find 41 studies where only 4 have the status “completed with results”, 14 are “completed without results”, 9 have “unknown status” and 6 are “withdrawn” or “terminated”
• Many nasal spray companies having to majorly walk back false claims of their sprays preventing COVID-19 after warning letters from the FDA (link here, ignore the Profi nasal spray praise, we’ll get to the study on it lol). As well as a lawsuit about falsely claiming to prevent COVID-19 when it comes to Xlear
• False claims that we mainly contract COVID-19 through nose cells (and not lung cells) with either no citation or citation of papers that don’t prove that (such as study E)
• Lack of acknowledgement that the location in the respiratory tract that aerosols end up is determined by their size (aka a nasal spray will not prevent the sizes of aerosols that end up in your lungs from going into your lungs), see Figure 3 and all the studies referenced in that figure in study F)
• Not everyone breathes through their nose
• Nasal sprays are flushed out of the nasal cavity in a matter of hours
• Nasal sprays don’t appear to coat even 50 % of the nasal cavity (see study G, study H, study I)
• Many of these sprays contain the preservative benzalkonium chloride, which have harmful effects at the concentrations used in nasal sprays in some studies (see study J and study K and references therein)
Note: the sizes of aerosols that would end up deposited in your nose are very efficiently filtered by high-quality respirators such as N95s, provided that the N95 is sealed to your face and the seal doesn’t break. This is even true for a respirator with a lot of wear time (see my previous post on some studies looking at the effects of wear time on N95 fit and filtration efficiency here, again, provided that it stays sealed). This is because the filtration mechanisms that act on the sizes of aerosols that get deposited in your nose do not degrade with wear time (whereas the filtration mechanisms that act on smaller aerosols do degrade with wear time). Thus, while wearing a sealed N95, aerosols containing SARS-C0V-2 in the environment should not be deposited in your nose anyway.
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Summary/TLDR and final thoughts
Unfortunately, many people including covid influencers have fallen for the falsehood that nasal sprays prevent COVID-19. Some such influencers have promoted these nasal sprays for free and helped spread the misinformation that they prevent COVID-19. Unlike with nasal sprays, there is ample, sound evidence that high-quality well-fitting respirators, ventilation and air purification prevent COVID-19.
The human clinical trials testing whether or not nasal sprays prevent COVID-19 have major methodological issues, and to my knowledge there are only two (EDIT MARCH 15, 2025: and one of the two has now been retracted)! Please don't lower your covid precautions based on two (EDIT MARCH 15, 2025: one) low-quality, flawed human clinical trials, two low-quality, flawed human studies with no placebos and other misleading studies performed in test tubes! As time goes on, more concerns about these studies appear on PubPeer which sometimes triggers investigations of the studies and warnings to not treat the studies as reliable in the meantime. Most clinical trials looking at preventing COVID-19 with nasal sprays mysteriously never published the results (most likely, the results were not good so they didn’t publish them). In my (PhD biochemist who studied COVID-19) opinion, we have enough studies to reasonably conclude whether or not nasal sprays prevent COVID-19, and we may not get many new ones, because the evidence suggests that nasal sprays do not prevent COVID-19. However, as a scientist, I will continue to review any new studies and keep an open mind.
While this post may be upsetting to read, false hope is dangerous. Well-fitting high-quality respirators, ventilation and air purification give me true hope. Many of these companies are no longer allowed to claim that their sprays prevent COVID-19 after warnings from the FDA. Let’s stop spreading dangerous misinformation and stop providing free advertising for these companies who have never proven their sprays prevent COVID-19! <3”

#MaskUp #WearAMask #CovidRealist #CovidIsAirbone #LongCovid #YallMasking #DisabledLiberation #DisabilityJustice

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