^Some public health experts have warned that health systems in North America and Europe will face a double challenge if the coming autumn and winter weather fuels both another wave of COVID-19 and the usual seasonal influenza epidemics. As well as the additional stress on health systems, people who get both infections back to back or at the same time may face added health risks.
Fortunately, the same public health and social measures that have been recommended to curtail COVID-19 also work exceptionally well to reduce influenza and some other respiratory infections. In an earlier Weekly Science Review, we presented flu surveillance data from countries that were affected by COVID-19 in March and April. After implementing COVID-19 responses, including mandated restrictions, many saw their influenza cases drop off weeks earlier than expected. In temperate countries of the Southern Hemisphere seasonal influenza usually occurs from May to September. In its most recent surveillance update, the World Health Organization (WHO) concludes that the Southern Hemisphere’s 2020 flu season hadn’t even started as of July 19, right when many countries expect to see case counts cresting. The image below shows seasonal influenza cases experienced in Chile for the first 28 weeks of each year since 2015, with barely any activity in 2020. Even in tropical countries where flu can occur sporadically year-round, influenza cases and positive samples reported to WHO have been sparse in recent weeks, and none at all were reported from tropical zones of Africa, South America, or Southeast Asia.
Seasonal influenza cases reported by epidemiological weeks 1 to 28, 2015–2020.
Chile's Influenza Cases During Flu Season
This may be an encouraging trend, if it truly reflects that efforts to mitigate or suppress COVID-19 are simultaneously reducing the burden of another potentially grave respiratory illness such as seasonal influenza. Furthermore, with less global travel, there may be less global circulation of influenza viruses. WHO warns, however, that at least part of what’s being observed may be a collateral effect of the COVID-19 pandemic. If fear or mandated restrictions deter many people from seeking treatment, influenza cases may go undetected and unreported. If hospitals and health systems are overburdened with COVID-19, they may not be able to test and report on influenza as effectively as they have in prior years. And there is another reason for those of us in the Northern Hemisphere to be concerned: each year epidemiologists and virologists monitor seasonal influenza in the south in order to anticipate the coming season in the north (and they learn from what happens in the north to anticipate the following season in the south). That includes important work to select viruses for the seasonal flu vaccine and to anticipate how extensive transmission is likely to be. With so little seasonal transmission happening now, it will be harder to prepare for the Northern Hemisphere flu season in the coming months and predict which strains are likely to circulate, if there is a widespread influenza season. For their part, vaccine manufacturers are gearing up to produce and ship record numbers of flu shot doses for the coming season. Ultimately, according to Johns Hopkins University’s Outbreak Observatory, “the severity of seasonal influenza in the Northern Hemisphere may depend heavily on how well countries are containing COVID-19.” Wherever widespread COVID-19 transmission is occurring, seasonal influenza is likely to spread rapidly as well. Wherever health systems are already stressed by COVID-19 cases, the added burden of influenza will compound the crisis.^
preventepidemics.org/covid19/science/weekly-science-review/august-1-7-2020/