Hello everyone and welcome to this week's blog post, in which I will be discussing the common cold, the flu and COVID-19. The common cold, the flu and COVID-19 are all illnesses caused by viruses, which affect the respiratory system. However, while they share many symptoms, they are different diseases.
The common cold is a viral infection that affects the upper respiratory system. According to a paper published in 2005, there are over 200 different viruses which can cause a cold, however, rhinoviruses are by far the most common (causing around 10%-40% of cases). Meanwhile, about 20% of colds are caused by coronaviruses (a group of viruses). These are viruses which have a crown-like appearance when viewed at high magnifications (due to glycoprotein spikes on the viral envelope).
After the virus enters the body, it infects cells in the nose (particularly in the adenoid area) and starts replicating. This is because these cells have a specific receptor (ICAM-1), which the viral can attach to. Interesting, the virus itself only infects relatively few cells (thus causing limited damage to the membranes inside the nose), so is not directly responsible for symptoms of the cold. Instead, these symptoms are due to the body's response.
Following infection, an immune response is stimulated, which results in inflammatory mediators being released. Inflammatory mediators (e.g. histamine) are molecules which cause blood vessels in the surrounding area to dilate and become leakier. They also cause mucus to be secreted from glands and sneezing and coughing reflexes. As a result, the viral particles can be carried by airborne droplets and infect another person.
As I mentioned above, the common cold is transmitted through airborne droplets which are released from infected people. Common symptoms of the cold include a cough, high temperature, sneezing, sore throat and runny nose. Unfortunately, there is no cure for the disease, however, its symptoms can be managed through drugs such as ibuprofen, paracetamol, nasal decongestants.
It should be noted that, as the common cold is always caused by a virus, antibiotics should never be used in the treatment of the common cold. As I've mentioned several times already on this blog, the overuse of antibiotics is incredibly dangerous, due to the production of antibiotic-resistant bacteria and the destruction of the resident human microbiome.
Currently, there is no vaccine for the common cold. This is largely due to the fact that there are so many circulating strains of cold. For example, according to Peter Barlow, an immunologist in Edinburgh, there are over 160 different strains of rhinovirus alone. Consequently, producing a vaccine which is effective against all of them is extremely difficult. Further, due to the speed at which viruses replicate, new genes are constantly being created due to mutations, so even more strains of the virus are being generated.
Nonetheless, researchers are still attempting to create a cure for the common cold. One way they are going about this is by sequencing all of the strains and then figuring out which genes are common to each one. They hope that if they can create a vaccine which stimulates an immune response to this common structure, then the vaccine would offer protection from all the strains of rhinovirus.
Another group of researchers are trying to create a 'vaccine cocktail'. This is where vaccines for many strains of the disease are mixed together so the patient has immunity against all of them. For example, this technique was used to create a vaccine against all three strains of polio.
Other groups are looking for completely new compounds to cure an infection, such as host defence peptides (which are found in humans) and possibly even other microbes which could attack viruses on our behalf.
Influenza, which is commonly known as 'the flu', is also a respiratory infection. It is caused by a group of viruses called influenza viruses. There are four categories of influenza viruses which infect vertebrates - alphainfluenzavirus, betainfluenzavirus, gammainfluenzavirus and deltainfleunzavirus. Of these, alphainfluenzavirus, betainfluenzavirus and gammainfluenzavirus all can infect humans.
Alphainfluenzaviruses, which are responsible for all flu pandemics (including the Spanish flu and the 2009 swine flu pandemic) are classified into subgroups based on the combinations the hemagglutinin and the neuraminidase proteins on the protein coat.
Influenza viruses cause symptoms by infecting epithelial cells lining the lung airways, before hijacking the cell machinery to replicate and invade adjacent cells. This stimulates an immune response, as phagocytes (macrophages and neutrophils) detect the virus and released cytokines (proteins which act as local hormones). These cytokines attract T lymphocytes to the lungs, which recognise infected cells due to the antigens presented on their cell surface membrane.
The T lymphocytes then destroy these cells, which causes damage to the lungs, making breathing difficult. Mucus also begins to accumulate in the lungs, so the coughing reflex is stimulated to clear your airways. Further, the T lymphocytes can also accumulate in lymph nodes, causing them to swell. Moreover, as part of the immune response, body temperature is increased in an attempt to stop the virus from replicating, usually by denaturing the enzymes involved in protein synthesis
Common symptoms of the flu include muscle pains, fever, cough, fatigue a runny nose and a sore throat. In contrast to the common cold, influenza can often lead to serious complications, including pneumonia, and worsen pre-existing health problems such as asthma. As such, influenza can be deadly, resulting in up to 650,000 deaths each year according to the World Health Organisation.
The treatment of influenza depends on whether the patient is part of a high-risk group or not. If not, then patients are advised to use paracetamol or ibuprofen to reduce the fever and treat muscle pain, drink lots of water to prevent dehydration, and stay at home if possible to reduce the spread of the virus.
Those in high-risk groups who show signs of deteriorations are often treated with antiviral medications, such as neuraminidase inhibitors. It is important to note that antiviral drugs should not be overused as, like antibiotics, the overuse of antiviral drugs also leads to resistance. Indeed, all circulating influenza viruses are resistant to adamantane drugs, due to their use in the past.
Unlike the common cold, vaccines do exist for influenza viruses. Each year, around February, the World Health Organisation analyses the circulating influenza viruses and assesses which strains of the virus are likely to be circulating in the winter (as the flu is most deadly in the winter, so that is when vaccines aim to prevent cases). From March, manufacturers use these recommendations to start production of new flu vaccines which are used the following winter. These vaccines are often 'vaccine cocktails', as they contain vaccines for multiple strains of the virus.
As influenza can be deadly, in the UK the winter flu vaccine is free for those in high-risk groups. This includes children, those aged over 65, pregnant women, health and social care workers, and those which medical conditions which make them more susceptible to the virus. As different factors, such as age, health and allergies, affect which vaccine can be given to each person, there is a range of vaccines available.
SARS-Cov-2 (which stands for severe acute respiratory syndrome coronavirus 2) is a virus which causes the respiratory infection responsible for the COVID-19 pandemic. The virus itself is a single strand of RNA wrapped in a viral envelope, which has 3 types of protein - spike proteins, envelope proteins and membrane proteins.
The virus spreads in much the same way as the common cold and flu - through airborne droplets. However, some research has also shown that SARS-CoV-2 can be transmitted asymptomatically, so it should be noted that airborne droplets are not the only form of transmission.
Once the virus enters the respiratory system, it attaches to ACE2 receptors on the epithelial cells lining the lungs and injects its genetic material. The instructions on this genetic material are then read by the cell and the cell produces more viruses, which are then released from the cell. Some particles also infect immune cells and trigger a 'cytokine storm', which is where too many cytokines are released and the immune system is overstimulated. Similar to influenza, this causes many T cells to gather in the lungs and kill epithelial cells, resulting in the destruction of lung tissue.
While SARS-CoV-2 is a type of coronavirus (the same group of viruses which cause the common cold), the symptoms it causes are much more common and severe. These symptoms include having a fever, cough, fatigue, headache and breathlessness. According to the World Health Organisation, around 15% of patients infected with SARS-CoV-2 will have severe symptoms. In addition, this virus is also more likely to result in pneumonia, which is where the lungs become infected by other bacteria, due to the damage the virus has done.
At the moment, there are several treatments for coronavirus which are being tried, including stem cell therapy and other medications. As the pandemic progresses and more research is carried out, it is likely that treatment of the disease will improve and medications will be proved effective. At the time of writing, one of the most promising medications is remdesivir. In one trial, remdesivir was shown to decreases recovery time by 4 days and decrease mortality rates by 4.8%.
Many pharmaceutical companies and research groups are and manufacturing vaccines for this virus. While none have been proven successful in clinical trials, the UK government has placed an order for 100 million doses of a vaccine developed by Oxford University and manufactured by AstraZeneca. If this vaccine is proven effective, then 30 million doses of the vaccine could be available by September, which would decrease the future impacts of the virus significantly.
P.S. Please note that the information in this article is correct at the time of writing, but may have changed by the time you are reading this.
Sources:
Eccles R. (2005). Understanding the symptoms of the common cold and influenza.The Lancet. Infectious diseases,5(11), 718–725. https://doi.org/10.1016/S1473-3099(05)70270-X
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