COVID-19's shifting impact: the changing relationship between infections and severe outcomes

A recent study published in the PLOS Biology Journal explored the dynamics of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection hospitalization (IHR) and fatality (IFR) ratios in England over 23 months.

Study: Dynamics of SARS-CoV-2 infection hospitalisation and infection fatality ratios over 23 months in England. Image Credit: AlexanderSteamaze/Shutterstock.com

Background

SARS-CoV-2 has globally increased morbidity and mortality rates. England witnessed a massive surge in hospitalizations and deaths after SARS-CoV-2 Alpha emerged.

Consequently, a national lockdown was imposed in January 2021 to curb social contact, with the concurrent implementation of a mass vaccination program.

As a result, coronavirus disease 2019 (COVID-19) cases, hospitalizations, and deaths declined sharply in early 2021. Restrictions were gradually eased after March 2021, and the pandemic re-entered a growth phase with the emergence of the SARS-CoV-2 Delta in April 2021.

All domestic restrictions were removed in July 2021, with society reopening to an extent unseen since the start of the pandemic.

Restrictions were not since re-introduced at a large scale, even when the prevalence was high late in 2021 and during the Omicron waves.

Evaluating the trends between infection levels and hospitalization rates can help inform public health agencies and governments to implement proportionate and appropriate restrictions. When IHR and IFR are accurate, severe outcomes could be forecast over the short term.

The study and findings

In the present study, researchers explored the dynamics of SARS-CoV-2 IHR and IFR in England over 23 months. They used data from the real-time assessment of community transmission (REACT)-1 study that conducted 19 cycles of surveys from May 2020 to March 2022. Persons aged five or older were contacted for participation and sent a self-administered swab test.

Data on COVID-19 cases, hospitalizations, deaths, and vaccinations were accessed from an official government website. The time lag from swab positivity to the occurrence of severe outcomes declined throughout the study.

There was a time lag of 19 days to hospitalization and 26 days to death during REACT-1 cycles 1-7 (May 1 to December 3, 2020).

During cycles 14-19 (September 9, 2021, to March 31, 2022), time lags were shorter at seven days to hospitalization and 18 days to death. Contrastingly, time lags were extremely long during cycles 8-13 (December 30, 2020, to July 12, 2021) at 24 days to hospitalization and 40 days to death. The IHR and IFR were estimated to be 2.6% and 0.67%, respectively, during cycles 1-7.

IHR was 0.76%, and IFR was 0.09% during cycles 14-19. The IHR and IFR were far lower for participants aged 64 or younger than those aged 65 or above during cycles 1-7 and 14-19.

The team compared the average IFRs and IHRs over four-week intervals to a baseline period (May 1 to November 11, 2020).

The average IFR was 1.68 and 1.31 times greater than the baseline in late November 2020 and January 2021, when SARS-CoV-2 Alpha accounted for 15% and 86% of cases, respectively. The average IHR and IFR reduced to 0.51 and 0.25 of baseline in April 2021, when 47% of the population had received at least one vaccine dose.

The average IHR and IFR were 0.84 and 0.43 of baseline in June-July 2021, respectively, when the Delta variant accounted for 99% of infections and 50% of the population had been double vaccinated.

IHR and IFR showed a steady decline from September 2021 and were sharply reduced in December 2021, when the proportion of booster vaccine recipients increased.

The mean IHR was 0.62%, and the average IFR was 0.06% by March 2022, when the Omicron variant caused over 99% of cases. The time lag between swab positivity and daily case numbers varied throughout the study and was three days, -7 days, and one day during cycles 1-7, 8-13, and 14-19, respectively.

The case ascertainment rate, defined as the proportion of cases identified with a positive test through mass testing, was 36.1% overall and varied throughout the study.

It increased from around 20% in July 2020 to 30% during August-December 2020, with a sharp surge between May and July 2021 and a steep decline between December 2021 and March 2022.

Conclusions

The researchers illustrated the temporal relationship between community prevalence of SARS-CoV-2 infection and severe outcomes.

They estimated SARS-CoV-2 IHR, IFR, and case ascertainment rates by assessing the differences in the swab positivity estimates and the time lag of COVID-19 cases, hospitalizations, and deaths.

The findings revealed a decline in SARS-CoV-2 infection severity over time in England. Community-based studies like REACT-1 can provide unbiased temporal estimates of infection levels, allowing for rapid detection of IHR or IFR changes.

Appropriate interventions can be implemented with early warnings when they are highly effective.

Written by

Tarun Sai Lomte

Tarun is a writer based in Hyderabad, India. He has a Master’s degree in Biotechnology from the University of Hyderabad and is enthusiastic about scientific research. He enjoys reading research papers and literature reviews and is passionate about writing.