Seasonal flu and COVID

In the afternoon briefing on Wednesday, 10 February 2021, the Chief Medical Officer, Dr Michael McBride, commented that the COVID-19 restrictions have “most certainly also had an impact on other viruses that we normally see at this time of year” (7:17 from start).

Dr McBride added: “This winter, as yet, there have been no outbreaks of flu; no outbreaks of norovirus, the winter vomiting bug; and no outbreaks of RSV, the respiratory illness that affects our children so badly each winter …”

What is seasonal flu?

Seasonal flu is a very common illness, caused by influenza viruses. It is spread by coughs and sneezes and can be highly infectious. It tends to be more severe than the common cold. The most common symptoms are a sudden high temperature, sore throat, cough, headache, tiredness and general aches and pains. Flu can also cause nausea, loss of appetite, runny or blocked nose, sneezing and cause you difficulty sleeping. The World Health Organisation explain that “in temperate climates, seasonal epidemics occur mainly during winter, while in tropical regions, influenza may occur throughout the year”.

Can flu be prevented?

Good personal hygiene is encouraged to prevent the transmission of influenza viruses. Avoid touching the eyes, nose or mouth; frequent hand washing (with soap and water, or alcohol-based hand rubs); covering coughs and sneezes with a tissue or sleeve; avoiding close contact with sick people; and staying home when sick.

Flu vaccine programmes aim to protect against infection by influenza viruses and reduce sickness, medical visits, hospitalisations, and deaths.

The World Health Organisation’s Global Influenza Surveillance and Response System makes recommendations for two different influenza vaccine formulations every year: one for the northern hemisphere (made in February/March), and one for the southern hemisphere (September/October). The three or four strains of influenza (H1N1, H3N2, and often two Type-B strains) most likely to attack during the next winter are chosen for inclusion in the vaccine. Influenza viruses evolve quickly, and one year’s vaccine is likely to be less effective the following year.

Every autumn, the flu vaccine is offered to people in Northern Ireland who are:

    • pregnant;
    • aged 65 or over;
    • have an illness or underlying health condition (including children from 6 months of age);
    • pre-school children aged two years and over;
    • children at primary school and in the first year of secondary school (year 8);
    • live in a residential or nursing home;
    • people who care for an elderly or disabled person;
    • frontline health and social care workers who provide direct patient care.

The 2020/21 flu vaccination programme was expanded to cover:

    • 50-64 year olds;
    • household contacts of those who received shielding letters during the COVID-19 pandemic;
    • staff in independent care homes.

How is seasonal flu monitored?

The Public Health Agency in Northern Ireland has a sophisticated evidence-gathering process in place to monitor the level of influenza in Northern Ireland each year. The number of General Practice and out-of-hours consultations for flu or flu-like illnesses are automatically extracted from health IT systems. Patient samples – usually nose and throat swabs – are sent for testing. This testing is enhanced by additional samples taken by 36 ‘Sentinel’ flu-spotter GP practices, which feed into Northern Ireland and UK-wide surveillance programmes.

Seasonal flu levels in 2020/21?

Between week 40 and week 20 (October through to mid-May), Northern Ireland’s Public Health Agency publishes a weekly/fortnightly Influenza Weekly Surveillance Bulletin.

According to the latest bulletin, since October 2020 and until 7 February 2021, no flu – or more general respiratory – outbreaks have been reported in Northern Ireland, and just one patient has been admitted to ICU with confirmed influenza (in November 2020).

General Practice and out-of-hours consultation rates for flu are lower than 2019/20. While the number of samples being tested has increased during 2020/21 (as the regional virology lab is testing for both influenza and COVID-19), the proportion of positive results is very low compared with 2019/20, as suggested by the chart above.

In contrast with previous years, there is a zero rate of respiratory syncytial virus (RSV) being detected in tested samples, as suggested by the chart below.

Why are flu rates so low?

Associate professor of emergency medicine at the Johns Hopkins University School of Medicine, Eili Klein, explains: “Everything we are doing to slow transmission of COVID-19, such as wearing face masks, frequent handwashing and physical distancing, should also reduce transmission of flu”.

In the same article, John Hopkins’ senior director of infection prevention, Lisa Maragakis, adds: “We commonly see flu spread in communities, schools, businesses and through travel each year, so these changes are likely keeping the flu away”.

Virologist Richard Webby at St Jude’s hospital in Memphis, Tennessee told Nature magazine that flu levels are low in some countries which “haven’t done such a good job controlling COVID … I don’t think we can put it all down to mask wearing and social distancing”. Webby suspects that the reduction in international travel has lessened its spread. The article also suggests that wider than normal flu vaccination programmes in some countries may also have contributed to lower infection rates.

However, Klein sounds a note of caution about influenza infection rates next year:

“Because of the current restrictions and precautions everyone is taking this season, far fewer people will be infected or exposed to the flu virus, and therefore won’t become immune to certain strains of the virus … so the number of people who may have more severe infections next year is likely to be greater because immunity will be lower.”

Excess winter mortality

As Dr McBride suggested, seasonal flu is prevalent in the winter season in this part of the world. This contributes to additional deaths each winter. The Northern Ireland Statistics and Research Agency (NISRA) calculates an “excess winter mortality” (EWM) figure which compares the number of deaths that occurred in a winter period (December through to March) with the average number of non-winter deaths occurring in the preceding August to November and the following April to July.

The most recent excess winter mortality figures are for 2019/20 (charted below). NISRA’s analysis concludes that the EWM for winter 2019/20 was approximately 600. In the last 10 winters, EWM has ranged from approximately 560 to 1,620. They note that “the 5,802 deaths in Northern Ireland in the four months of winter 2019/20 (December to March) is the third highest number of winter deaths in the last 10 years”.

The COVID-19 pandemic has increased the number of deaths in non-winter months — April to July 2020 in particular — and thus decreased the difference in total deaths in the winter and non-winter periods, and lowered the EWM figure. NISRA comments:

“The true impact of Covid-19 on mortality in Northern Ireland is still unfolding, but the increase in deaths in the late March–July 2020 period has impacted on the EWM and EWMI results by decreasing the difference in total deaths in the winter and non-winter periods. We can estimate this impact on excess winter mortality in 2019/20 by removing all deaths where Covid-19 was the underlying cause of death from this analysis and re-calculating EWM. Forty-seven deaths occurred where Covid-19 was determined to be the underlying cause of death from December 2019 to March 2020 and 727 from April to July 2020. Removing these from the calculation would lead to an excess winter mortality of approximately 910.”

The excess winter mortality figure for 2020/21 would normally be expected to be published in November 2021 but NISRA cannot confirm the publication date of the next update due to uncertainty in the prioritisation of resources while the pandemic is ongoing.

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