14 August 2020
Epidemiologist Simon Thornley has reassured people that there has been no new information that concludes masks are warranted against COVID19.
“The trials are clear; there is no statistically useful safety gain in wearing a mask, although low quality observational studies show an advantage to wearing one in confined spaces.”
Thornley says he was moved to clarify the science because people advocating masks were claiming ‘things had changed’, leading to an increase in public mask-wearing since Wednesday.
“Nothing in medical science has changed since Ashley Bloomfield first told us months ago that there was no advantage to masks. What has changed is social and political advantage in advocating masks, and that’s not science.
“I will not be wearing a mask without symptoms, but anyone with compromised health could choose to carry a mask to wear in places like public transport.”
Thornley urged all public communicators, including media, to reference the latest science on masks so people could decide for themselves (see below).
On Monday, the Covid Plan B group is live streaming a COVID-19 Science and Policy Symposium, featuring international experts analysing the New Zealand situation. Find out more here.
The Science on masks
A trial in Australia showed that in households exposed to children with respiratory symptoms and fever, there was no difference in outcome between households that wore either surgical or P2 masks with controls who did not wear masks. The incidence of laboratory confirmed infections were twice as high in the mask wearing groups compared to controls, but the difference was not statistically significant.1 About 50% of patients reported problems with the masks, and by day five, only 30% of participants were compliant with the mask use.
A larger trial in Thailand that compared control, to handwashing, and handwashing and surgical masks in households with influenza-like illness showed no difference between the three groups, in terms of reducing the incidence of secondary transmission from primary cases.2
Meta-analyses of observational studies have reported benefits of reductions in risk of using masks.3 A case-control study of H1N1 transmission following a prolonged flight between China and the US showed a very strong association between mask use and protection from infection. None of the 9 cases wore masks, compared to 47% (15/32) of control passengers.4
The World Health Organisation5 only recommends masks when individuals have symptoms compatible with Covid-19. In populations such as those with a high prevalence of Covid, immunocompromised patients or times where high population density cannot be avoided, such as mass gatherings, public transportation (including aeroplanes), masks are considered useful for ‘source control’, rather than ‘protection’.
- MacIntyre CR, Cauchemez S, Dwyer DE, et al. Face mask use and control of respiratory virus transmission in households. Emerging infectious diseases 2009;15(2):233.
- Simmerman JM, Suntarattiwong P, Levy J, et al. Findings from a household randomized controlled trial of hand washing and face masks to reduce influenza transmission in Bangkok, Thailand. Influenza and other respiratory viruses 2011;5(4):256-67.
- Chu DK, Akl EA, Duda S, et al. Physical distancing, face masks, and eye protection to prevent person-to-person transmission of SARS-CoV-2 and COVID-19: a systematic review and meta-analysis. The Lancet 2020;395(10242):1973-87. doi: https://doi.org/10.1016/S0140-6736(20)31142-9
- Zhang L, Peng Z, Ou J, et al. Protection by face masks against influenza A(H1N1)pdm09 virus on trans-Pacific passenger aircraft, 2009. Emerging infectious diseases 2013;19(9):1403-10. doi: 10.3201/eid1909.121765
- Organization WH. Advice on the use of masks in the context of COVID-19: interim guidance, 5 June 2020: World Health Organization, 2020.