Implications of the SARS-CoV-2 pandemic on influenza epidemics

By Shanet Susan AlexReviewed by Aimee MolineuxFeb 9 2022

A recent study posted to the medRxiv* preprint server investigated the effect of low circulating seasonal influenza virus during the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic on the antibody titers against influenza during the same period.

The study further explored the impact of antibody immunity against the influenza virus during the coronavirus disease 2019 (COVID-19) pandemic in future influenza epidemics.

*Important notice: medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Background

Seasonal influenza viruses are usually associated with yearly epidemics causing infections in around 5 to 15% of the population worldwide. However, during the initial two years of the COVID-19 pandemic, the documented seasonal influenza cases were fewer, along with unprecedentedly lower circulating rates. As of January 2022, the number of influenza cases reported to the World Health Organization (WHO) was over 80% less than the historical averages of seasonal influenza cases.

Once the non-pharmaceutical interventions for COVID-19 are lifted, a potential threat of seasonal influenza and severe influenza epidemics exists because of the lack of immunizations against seasonal influenza during the COVID-19 pandemic and waning antibody titers from previous influenza infections.

Studies conducted before the COVID-19 pandemic have demonstrated that antibody titers towards influenza A virus typically wane by half within 3.5 to 10 years after the infection. However, evidence for antibody titers against the influenza virus during the SARS-CoV-2 pandemic when seasonal influenza infection has been nearly absent, as well as its future impacts, are yet to be understood.

About the study

In the current study, the researchers determined the impact of low circulating seasonal influenza virus infections during the COVID-19 pandemic on the antibody titers against the influenza virus and the size of the future influenza epidemic. The hemagglutination inhibition (HI) of the representative seasonal influenza viral strains such as B/Victoria, A/H3N2, B/Yamagata, and A/H1N1pdm09 in 630 longitudinally collected sera samples from 165 adults in the Netherlands before and during the SARS-CoV-2 pandemic were used to quantify the antibody titers.

The study samples were collected from two independent cohorts; 1) 100 participants of the Amsterdam cohort studies (ACS) on human immunodeficiency virus (HIV) infection and acquired immunodeficiency syndrome (AIDS), constituting 500 sera samples and 2) 65 subjects of a prospective cohort study with recovered SARS-CoV-2 patients, constituting 130 sera samples.  

Results

The results indicated that the mean HI titers of the cohort of all types and subtypes of seasonal influenza virus were higher after the influenza epidemic of 2017 and 2018. However, by the summer of 2019, the mean cohort HI titers were at the pre-2017/2018 levels. Mean HI titers remained stable from 2019 through 2021, during the COVID-19 pandemic when the circulation of the influenza virus was insignificant.

The assessment of HI titers in ACS participants against A/H1N1pdm09, B/Yamagata, and A/H3N2 viruses indicated that waning of immunity against seasonal influenza viruses occurs at timepoints even longer than the pause in the circulation of seasonal influenza virus observed during the initial two years of the SARS-CoV-2 pandemic. Further, the evaluation of HI titers in ACS participants from 2020 to 2021 indicated no significant waning of immunity to any of the influenza viruses studied.

Similarly, the analysis of sera samples of COVID-positive adults from 2020 to 2021 against the B/Yamagata, B/Victori, A/H3N2, and A/H1N1pdm09 viruses demonstrated only a negligible loss of immunity towards the same viruses, given the subjects were not vaccinated against the seasonal influenza virus in 2020.

Year-on-year individual HI titer distributions stratified by the increase in titers indicating recent influenza virus infection demonstrated that influenza A and B viral infections were frequent in individuals with low antibody titers during the year before influenza infection.

Further, the low country-level prevalence of each subtype of influenza virus during one or more years had only a minor effect on the size of the next epidemic, according to the epidemiological data of the historical influenza virus from 2003 to 2019.

Conclusions

The study findings suggested that the waning of antibody titers against seasonal influenza viruses over the initial two years of the COVID-19 pandemic was insignificant indicating that the risk of the seasonal influenza virus remained significantly unchanged during these years.

The stable antibody titer against the influenza viruses during the first two years of the COVID-19 pandemic period and about 20 years of epidemiological data regarding seasonal influenza virus suggested that the near absence of the circulating seasonal influenza viruses during the SARS-CoV-2 pandemic was associated with a low potential for significantly larger epidemics of seasonal influenza in the future.

Further, the study suggested that the size of the future seasonal influenza virus epidemics might be comparable to that of the seasonal influenza epidemics observed before the SARS-CoV-2 pandemic.

*Important notice: medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.