Overview

Influenza ( commonly known as “the flu”) is an infection caused by the influenza virus. It causes different symptoms, including but not limited to fever, coughing, a sore throat and fatigue. Despite being common, it is generally not seen as serious in normal circumstances, as these symptoms can take as little as a few days to disappear.

There are, however, different factors that affect the severity of influenza symptoms. These could be genetics, a previous condition or age – the last being quite varied.

Influenza is often mistaken to be the same as the common cold which produces similar symptoms (except for a runny nose) however, the common cold can be caused by multiple types of virus, including influenza.

Newborns and infants

Underdeveloped immune responses

Despite being fully formed physically after delivery, newborns are extremely vulnerable to the outside world. This is because their immune systems are weaker and less developed than teenagers and adults, making babies vulnerable to more serious consequences of influenza.

The innate immune response in our bodies is considered the first line of defence when pathogens like viruses enter our bodies. In a typical response to the influenza virus, dendritic cells produce interleukin-12, a signalling molecule that induces a T-helper cell response [Fig. 1]. Then, killer T cells are produced that destroy any cells infected with the virus to prevent its spread to other cells

Figure 1: Flow diagram illustrating the signalling process within the innate immune system. These steps form what is known as a pathway which is common in many molecular processes ensuring regulation. Newborns and infants naturally produce less interleukin-12, which has a lesser effect on the following steps within the innate immune response pathway.

The dendritic cells of newborns naturally produce less interleukin-12 than older people, thus the chain of signalling that forms the immune response to influenza either is less likely to occur or is too weak to fight the infection.¹ Influenza is therefore significantly more serious in newborns than a typical child or adult whose innate immunity has developed over time.

Respiratory issues

Influenza can infect the lungs, which is why respiratory symptoms are common, like coughing or shortness of breath in some. Research has shown that immune molecule production is predominantly unresponsive in the presence of influenza in newborns; this leaves them extremely vulnerable to severe damage from the infection.² If infected, newborns are highly likely to undergo hospitalisation and there is the chance of death.

Vaccinations

Babies up to 6 months old are encouraged to get the influenza vaccine to minimise the risk of contracting influenza and experiencing its harsh effects.

Before being born, the expecting mother may be offered an influenza vaccine if she has had one before. Hospital staff that will be directly involved in the pregnancy (nurses, doctors, OB/GYN) would also be vaccinated to further minimise risk.¹

Children

Ease of transmission

While not necessarily a severity factor, children are more likely to get influenza from other people, typically other children.

There are two main causes: the first is that children are less likely to stick to hygiene etiquette than those older than them, such as not covering their mouth or nose when coughing or sneezing. The second is environmental; children are usually surrounded by other children within the school, which makes the speed of potential transmission very high.

Cardiac and respiratory complications

Influenza has the potential to give rise to complications, additional conditions that together with the flu can be critical to overall health.

Such conditions may exist before getting influenza, even from birth. Children who have congenital heart disease have noticeably high case numbers of hospitalisations from influenza.³ This is most common with children up to 2 years old and generally affects younger children (7 years and younger) [Fig. 2]. Across 5 different years of data collected, children assigned male at birth (AMAB) with influenza hospitalisation than those assigned female at birth (AFAB).

Figure 2: Graph displaying the number of cases of hospitalisations associated with influenza in young people, aged 0 to 20 years old. Data was taken across 5 different years. CHD = congenital (from birth) heart disease.

Children with respiratory conditions also appeared to be vulnerable to influenza hospitalisation, with 28% of all children having a history of asthma. This includes those with congenital respiratory conditions however, this is a very small proportion out of all the children hospitalised. Children with congenital respiratory conditions tended to also have congenital heart problems making their situations much more severe than the other children recorded in the study. Approximately 5% of children displayed respiratory failure.³

The number of cases in 2009 data is significantly higher than in other years, which is likely due to the swine flu pandemic that made its way to the UK. Mortality rates across all years within this study were low due to the availability of adequate medical care.

This data shows that there are potentially severe consequences if children are to contract the flu in the presence of other conditions.

Neurological complications

the possibility of complications due to pre-existing conditions, influenza can also cause complications if you have a history of additional conditions, even if you do not have them currently.

In another study, collecting data on influenza hospitalisations from 2013 to 2018, over 65% of child patients had neurological complications, with the majority of them aged between 1 and 5 years old.⁴

Among 67 children who were influenza-hospitalised with neurological complications (IHNC), over one-third displayed signs of febrile seizures or epilepsy. Two-thirds of all IHNCs had at least one seizure with three experiencing status epilepticus, continuous and uninterrupted seizures or a single seizure that can last as longer than 5 minutes.

The data shows that influenza has the potential to increase the severity of neurological (predominantly seizure-associated) conditions in very young children.

Pregnant and AFAB (assigned female at birth)

People assigned female at birth (AFAB) who are expecting a child have an increased likelihood of being hospitalised. Aside from any actual complications present, this is typically to assess the perinatal health of the foetus. Despite increased severities in pregnant AFABs, adequate medical intervention is available such as caesarean delivery, which is required more in mothers with influenza than those without. Assisted vaginal delivery may also be considered, but this is on a case-by-case basis.⁵

Seasonal flu

Complications and the necessity of extended hospitalisation can be reliant on vaccination status. The Influenza virus is capable of mutating its surface receptors so that our bodies do not initially recognise it, allowing the virus to cause infection before the immune system can act. This is referred to as seasonal flu, and it is recommended that vaccines (commonly known as flu shots) are taken annually.

While contracting seasonal flu is not particularly dangerous for mothers, lung-related issues have been reported. These include pneumonia, a bacterial infection and adult respiratory distress syndrome whereby the lungs are unable to sufficiently provide oxygen for the bloodstream.⁶ Extracorporeal membrane oxygenation is a type of external life support that can provide oxygen to the mother because if left untreated, there is a risk of cardiac arrest or stillbirth.

Whilst expecting mothers cannot be defined by an age range, this data should be assumed to apply to AFABs between mid-20s to late 30s, the age range in which fertility is most likely (and statistically most common).

Elderly

Approaching old age is associated with the body not working as strongly as it used to, and this includes immune responses too. immunosenescence refers to the deteriorated state of elderly immune systems, which affects how their bodies try to fight off influenza.

The elderly are vulnerable to new strains of influenza, with hospitalisation rates for those 65+ years old being higher specifically when the influenza A/H3N2 strain is present. Vaccination is typically available and recommended in preparation for the oncoming strain.⁷

Five severe complications have been identified to be common with influenza in the elderly:

• Stroke
• Pneumonia 
• Congestive heart failure
• Cancer
• Ischaemic heart disease

Some of these conditions such as stroke and ischaemic (restricted blood flow) heart disease are already likely to occur in some populations of the elderly, but the risk is higher if influenza is present. These conditions often lead to physical disability which significantly limits independence that could lead to further hospitalisation or another form of round-the-clock care such as assisted living.⁸

The detrimental effects of compounded medical problems can occur with a condition existing before influenza infection. Elderly individuals (65 to 80 years) with chronic (long-lasting) conditions such as congestive heart failure, coronary artery disease and diabetes mellitus can undergo influenza-associated hospitalisation.⁷ While case numbers for this are not as common as within middle-aged (40 to 65 years) counterparts, they are more likely to carry more severe consequences, much like when complications arise post-influenza infection.

Summary

There is no direct reasoning linking the severity of the flu with how old you are, but certain events or conditions are just more likely to happen at certain ages. Generally, the complications that arise do not tend to be commonplace, thanks to adequate healthcare and, more specifically, vaccines. The chances of the worst outcomes of issues compounded with influenza: death and disability can be avoided with periodic flu shots and close hospital monitoring.