Symptoms and respiratory infections: Insights from the UK 2022/2023 winter season

Ever wondered about the likelihood of having the flu if you’ve got a fever? Or how your age influences the symptoms you experience if ill with RSV? And can we distinguish SARS-CoV-2 from other respiratory infections based on which symptoms people report? These are some of the questions our recent research study explored. In this blog post, I’ll explain the motivation behind our study and highlight our key findings. 

What was our study about?
Respiratory infections affect lots of people every winter, and cause symptoms such as sneezing or fever. Often, these symptoms are mild, but such infections can also lead to more serious illness. Understanding how many people are getting different respiratory infections over time, and what symptoms they experience, is important to accurate diagnosis and effective treatment in the NHS. Getting more knowledge about these infections can also aid healthcare preparedness and inform vaccination policies.

Influenza A/B (flu) and respiratory syncytial virus (RSV) are two common causes of respiratory infection. Since the start of the COVID-19 pandemic, SARS-CoV-2 has also been co-circulating with these viruses. Our recent study investigated these three respiratory infections in the UK during the 2022-2023 winter season, along with self-reported symptoms. This study was based on a large household survey, the Office of National Statistics COVID-19 Infection Survey. Around 750 participants in the survey were randomly selected each week to have their nose and throat swabs PCR-tested for flu, RSV, and SARS-CoV-2. Our research study analysed this data using a range of statistical methods, including generalised additive models, which is a type of regression model.

What were the main findings?
Our study found that differentiating between flu, RSV, and SARS-CoV-2 based on symptoms alone may prove difficult. Symptom profiles were largely similar across the three viruses, with middle-aged participants being more likely to report symptoms than other age groups. Cough, sore throat, sneezing, and fatigue were amongst the most common symptoms for all three infections.

We did however see some differences between the three infections. For instance, RSV-positive people in our study generally reported fewer symptoms than those who tested positive for SARS-CoV-2 or the flu. In particular, fever was relatively uncommon amongst those with RSV. Importantly, we also observed considerable background rates of several symptoms, especially cough in young children, and fatigue and headache in middle-aged participants.

SARS-CoV-2 positivity was notably higher in adults than children, and RSV was more common in children below school year 12 than in older participants. Flu was also less common in participants aged 50 years and above than in younger age groups.  By estimating incidence over time using a technique called Richardson-Lucy deconvolution, we found that flu peaks occurred earlier in children than older adults, with a delay of around 2 weeks.

Many of the symptoms reported by people in the survey could not be explained by either of the three viruses we studied, highlighting the role of other infections such as the common cold. Fever was the symptom with the highest percentage of test-positive participants. The likelihood of testing positive for either of the three viruses generally increased by age across symptoms. However, even in older individuals reporting influenza-like-illness as defined by the WHO, only around ~30% had a positive test result for either SARS-CoV-2, the flu, or RSV. We also found that reporting flu vaccination in the past two winter seasons was associated with a lower likelihood of testing positive for flu in the current season.

What are the implications?
Given our findings, further research is needed on the role of other infections such as rhinovirus in causing respiratory symptoms. Future studies would benefit from having more specific data about the onset timing of symptoms and infections. However, our study emphasises the value of community-level data to understanding trends in respiratory infections and symptoms in people outside healthcare settings. Our study also raises important questions about the role of age in infection susceptibility, illness natural history, symptom reporting behaviour, and vulnerability to other symptom-inducing conditions. These questions provide promising avenues for future research, and highlight important considerations to the NHS and public health more broadly.