Flu vaccine reduces influenza risk with no effect on coronaviruses
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Influenza vaccination reduced the risk for influenza by more than 40%, with no effect on coronaviruses or other non-influenza respiratory viruses, according to a study assessing seasonal influenza and coronaviruses over seven seasons.
“Vaccines induce specific antibody protection, targeting the viral or bacterial antigens that are included as vaccine components. On that basis, influenza vaccines are expected to reduce the risk for illness due to influenza viruses and have no effect on other non-influenza respiratory viruses (NIRV),” Danuta M. Skowronski, MD, FRCPC, of the British Columbia Centre for Disease Control and the University of British Columbia in Vancouver, told Healio. “Recently, however, a paper published in the journal Vaccine suggested that receipt of influenza vaccine may adversely affect other NIRV and in particular may be associated with an increased risk for illness due to seasonal coronaviruses.”
To assess this, Skowronski and colleagues retrospectively applied test-negative design (TND) analysis to specimens from the Canadian Sentinel Practitioner Surveillance Network influenza vaccine effectiveness study that were obtained during the 2010-2011 through 2016-2017 seasons, when specimens were tested for both influenza and NIRV, according to the study. The researchers included specimens that were collected between November and April from consenting patients aged one year or older who presented to a sentinel provider within 7 days of influenza-like illness onset in the provinces of Alberta, British Columbia, Ontario or Quebec. Participants who self-reported receiving the influenza vaccine at least 2 weeks before the onset of influenza-like illness were considered vaccinated.
Overall, 4,281 influenza, 2,565 NIRV and 3,841 pan-negative specimens were included, with 175 coinfections identified through sensitivity analyses. According to the study, NIRV included enteroviruses/rhinoviruses (645; 25%), coronavirus (570; 22%), respiratory syncytial virus (524; 20%), human metapneumovirus (390; 15%), parainfluenza (316; 12%), adenovirus (114; 4%) and bocavirus (6; <1%).
The OR for influenza vaccination among influenza cases compared with influenza test-negative controls was 0.55 (95% CI, 0.5-0.61), corresponding to a vaccine efficacy of 45% (95% CI, 39%-50%). ORs were comparable when pan-negative (0.58; 95% CI, 0.52-0.65) or NIRV-positive controls (0.51; 95% CI, 0.45-0.58) were used. ORs were also similar when participants who were vaccinated less than 2 weeks prior to the onset of influenza-like illness were considered unvaccinated (0.56; 95% CI, 0.51-0.62).
The authors noted that influenza vaccination had no significant effect on any NIRV studied either separately or in combination, including during sensitivity analyses. The study showed that the OR for influenza vaccination among coronavirus cases compared with coronavirus test-negative controls was 1.04 (95% CI, 0.85-1.28), with similar results when participants were vaccinated less than 2 weeks prior to the onset of influenza-like illness and therefore considered unvaccinated (OR = 1.04; 95% CI, 0.85-1.27).
The researchers observed no difference in ORs for vaccine effect between participants aged less than 20 years (OR = 0.56; 95% CI, 0.44-0.70) and participants aged 20 years or older (OR = 0.55; 95% CI, 0.49-0.61). Vaccination did not significantly impact coronavirus risk in either age group.
“Influenza vaccination did not increase the risk for illness due to seasonal coronaviruses,” Skowronski said. “These findings were robust across multiple NIRV outcomes and sensitivity analyses.”
Findings from the current study demonstrate that influenza vaccination will not negatively affect one’s risk for COVID-19, she continued.
“Addressing such speculation is important to maintain influenza vaccine coverage through the COVID-19 pandemic, which expected to extend into the 2020-2021 influenza season in the northern hemisphere,” Skowrowski said. “In particular, influenza vaccination will reduce the risk of febrile cough illness due to influenza viruses and alleviate some of the seasonal health care burden during periods that influenza and NIRV, including SARS-CoV-2, cocirculate.”
The results also reinforce the idea that valid TND estimates that require a cause for which there is an effective vaccine be specifically excluded from the test-negative control group, Skowronski said, noting that this also applies when examining a vaccine’s effects on non-vaccine target pathogens.
“When assessing influenza vaccine effects against non-influenza causes of febrile respiratory illness (eg, coronaviruses), investigators must take care to exclude influenza cases from the control group,” she said. “These methodological insights have important implications for other TND applications, including future evaluations of influenza vaccine effects against COVID-19 and vice versa when SARS-CoV-2 vaccines become available.”
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