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Carrageenan containing nasal spray has proven to be safe and effective. But I think that the key point is that proper hygiene of the upper respiratory tract is indeed effective (especially with a virus that takes 5-6 days to incubate there), both to prevent infection progress and transmission.
There’s a lot of in vitro and in vivo evidence (again, check the bibliography in the already linked above article, but just to mention clinical trials (small, pilot mostly):
Beneficial effects of a mouthwash containing an antiviral phthalocyanine derivative on the length of hospital stay for COVID-19. https://doi.org/10.21203/rs.3.rs-330173/v1
Regarding the clinical trial, the median LOS [Length of hospital stay] of the AM [active mouthwash] group was
significantly shortened (4 days) compared with that of the NAM [No active mouthwash] group (7
days) (p=0.0314). Additionally, gargling/rinsing with APD was very
helpful in reducing the severity of symptoms (no ICU care was needed)
compared to not gargling/rinsing with APD (28.6% of the patients in the
NAM group needed ICU care, and 50% of this ICU subgroup passed way,
p=0.0207).EARLY VIRAL CLEARANCE AMONG COVID-19 PATIENTS WHEN GARGLING WITH POVIDONE-IODINE AND ESSENTIAL OILS – A CLINICAL TRIAL. https://doi.org/10.1101/2020.09.07.20180448
Five confirmed Stage 1 COVID-19 patients were recruited for each arm.
The age range was from 22 to 56 years old. The majority were males. Two
respondents had co-morbidities, which were asthma and obesity. Viral
clearance was achieved at day 6 in 100%, 80%, 20% and 0% for 1% PVP-I,
essential oils, tap water and control group respectively.A randomized trial on the regular use of potent mouthwash in COVID-19 treatment. https://doi.org/10.1101/2020.11.27.20234997
Our findings showed statistically significant improvement in terms of a
higher conversion rate to “COVID19-negative PCR” by five days of
treatment (6/46 Vs 0/46), improvement in “symptoms severity” after two
days of treatment, and less intubation and mortality (0/46 Vs 3/46) with
all P-value < 0.05.All these are with persons already infected. If used for prophylaxis the effectiveness should be very high (together with other measures, like air disinfection -see linked article-) it would have stopped this virus from spreading back in March 2020.