COVID-19 Vs. Influenza: Influenza Vaccination Amid COVID-19 Pandemic

Severe acute respiratory syndrome–coronavirus 2 (SARS-CoV-2), a highly contagious virus, emerged in 2019 from Wuhan, China (1). It rapidly spread around the world causing a novel acute respiratory disease, coronavirus disease 2019 (COVID-19). The World Health Organization (WHO) declared COVID-19 a pandemic on March 11, 2020. Consequently, the current COVID-19 pandemic impacts global health and economies to unprecedented levels. As of August 17, 2020, over 21,760,000 cases have been confirmed in more than 188 countries, with over 776,580 deaths, and growing daily. The spectrum of disease with SARS-CoV-2 ranges from asymptomatic infection to severe, often fatal disease. Patients with mild disease (80%) have fever, cough, sore throat, loss of smell, headache, and body aches (2). A surge of COVID-19 patients resulted in enormous challenges for capacity and patient flow in hospitals and health care systems globally. Currently, we have limited interventional strategies in curbing COVID-19, and attention has been focused on the progress in the development of vaccines and therapeutics since the beginning of pandemic. Despite the progress, one cannot exclude that the virus would be continuously circulating as a seasonal virus even after the availability of a vaccination program.

Seasonal influenza is a major cause of morbidity, mortality, resulting in a burden on  healthcare services globally every year. According to the WHO, up to 650,000 deaths are associated with seasonal influenza respiratory infections annually. In the Northern Hemisphere, the 2020-2021 influenza season will coincide with the continued circulation of SARS-CoV-2. The nature of disease similarity between COVID-19 and influenza is cause for great concern. In addition, SARS-CoV-2 and influenza viruses have similar transmission characteristics. The two viruses are spread by contact and airborne transmission. The incubation period for influenza is short, typically 1–2 days, whereas for SARS-CoV-2, it is 4.5–5.8 days (2). The basic reproductive rate (R0, the average number of secondary transmissions from one infected person) for SARS-CoV-2 is estimated to be 2·5 (range 1·8–3·6) compared with 2·0–3·0 for the 1918 influenza pandemic, 1·5 for the 2009 influenza pandemic, and 1.3 for seasonal influenza viruses (3, 4). COVID-19 mortality risk has been highly concentrated at old ages (> 65 years old) and those, in particular, males, with underlying medical conditions (called co-morbidities), including hypertension, diabetes, cardiovascular disease, and immunocompromised states (2). Furthermore, SARS-CoV-2 can also infect younger individuals. In particular, children have shown to be susceptible to infection (5). Although most of the infections run a rather benign course, some children may develop severe primary and unique secondary inflammatory complications of infection, including multisystem inflammatory syndrome of children (6). Indeed, while children comprise 22% of the U.S. population, recent data show that 7.3% of all cases of COVID-19 in the U.S. reported to the Centers for Disease Control and Prevention (CDC) were among children (as of August 3rd, 2020). The number and rate of cases in children in the U.S. have been steadily increasing from March to July 2020, even though the incidence of SARS-CoV-2 infection in children is known to be underrated due to a lack of widespread testing. Opening schools in many locations might change a dynamic of transmission of SARS-CoV-2 and COVID-19 cases among children. Similar to COVID-19, influenza-associated excess mortality in elderly individuals related to a range of other chronic health conditions, including cardiovascular causes, diabetes, neoplasms and renal disease (2). In contrast to COVID-19, children are believed to have the highest rates of infection and complications arising from influenza, thus leading to high rates of excess outpatient visits, hospital admissions and antibiotic prescriptions (7). Infections among children can also drive influenza epidemics due to their increased susceptibility to infection and greater contribution to the spread of virus in the community.

Vaccination can be the most efficient and effective measures in controlling the current COVID-19 pandemics. Researchers are developing more than 170 vaccines against the coronavirus, and 47 vaccines are in human trials. In contrast, annual influenza vaccination is available with inactivated influenza vaccines, recombinant influenza vaccine, and live attenuated influenza vaccine. This the main public health intervention in reducing the burden of disease (8). The WHO has recognized some priority target groups for annual influenza vaccination, including pregnant women, children aged 6 months to 5 years, the elderly, subjects with specific chronic conditions, healthcare workers, and international travelers (9). However, influenza vaccination rates among children aged 6 months to 17 years remain low compared with other routinely recommended childhood vaccines. In-plan vaccination coverage during the 2016–17 season was 67.7% in infants (born 2015), 49.5% in toddlers (born 2012–2014), 35.0% in school-aged children (born 2004–2011), and 22.3% in teenagers (born 1999–2003) (10). Like vaccination coverage, vaccination opportunities decreased with age. Along with continued efforts to reduce missed opportunities, effective strategies to bring children to their doctor for annual influenza vaccination are needed, particularly for older children. Among adults, influenza vaccination coverage (≥18 years) was 45.3% in the U.S. during 2018–19 influenza season (11).

The information regarding COVID‐19 and influenza coinfection is limited. Unless screening patients with COVID‐19, the coinfection remains undiagnosed and underestimated. The severity of disease resulting from the co-infections varies by causing a more severe course with a fatal outcome or mild illness (12). Although this needs to be further evaluated, influenza immunization for high-risk groups can reduce the possibility of influenza infection and co-infection with SARS-CoV-2 and complications associated with diagnostics and antiviral treatment. A COVID-19 infection prediction model has also shown that influenza vaccines could reduce COVID-19 infection risk (13). This will also alleviate burden on the health care system by avoiding an overload of health services and hospitals associated with influenza infections (i.e., outpatient illnesses, hospitalizations, and intensive care unit admissions). Influenza vaccine is safe for elderly and children with a proven record over the past 50 years (7). Therefore, influenza vaccination can be a critical component of response to the COVID-19 pandemic. However, there has been a prediction that the COVID-19 pandemic could decrease influenza vaccination, since the pandemic resulted in a 38 percent drop in consumer spending on health care and loss of health insurance (14). In response, CDC already arranged for an additional 9.3 million doses of low-cost flu vaccine for uninsured adults, up from 500,000. The agency expanded plans to reach out to minority communities. It is uncertain how this upcoming influenza season will evolve under the current circumstance. In general, taking an influenza vaccine can be a good preventive strategy for public health.


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