Case studies for viruses

Case study: rapid evolution in HIV

Human Immunodeficiency Virus (HIV) is a Retrovirus that was first identified in the early 1980s. The virus is transmitted through infected body fluids and primarily infects 

CD4+ cells

 of the immune system, leading to the disease Acquired Immunodeficiency Syndrome (AIDS). AIDS is now considered to be a global pandemic, with millions of people worldwide living with the disease. Although there are preventatives available, as well as treatments to slow the progress of the disease, there is currently no cure for AIDS, in part due to the high rate of evolution in the HIV virus.

HIV reproduces very quickly, and each time it does so, there is a chance that mutations will occur in its genetic material. HIV is an RNA virus, which means that reproduction of the genetic material requires the use of reverse transcriptase, an enzyme that is considered to be many times more error-prone than DNA polymerases. These mutations can lead to the emergence of new HIV strains that may be more or less 

virulent

, more or less transmissible, or more or less able to evade the immune system or antiviral drugs. HIV is also able to carry out recombination, exchanging genetic material with other viruses or its host cell, acquiring new genes and traits which can further increase its adaptability.

HIV transmission and infection is covered in more detail in sections C3.2.11 and C3.2.12.

Case study: rapid evolution in influenza

Like HIV, influenza or the ‘flu’ is an RNA virus, meaning that it has a less accurate means of replicating its genetic material than DNA viruses or living organisms. High mutation rates in the 

haemagglutinin

 and 

neuraminidase

 surface proteins can make it more difficult for the host immune system to identify and protect against the virus, and is also a contributing factor to why flu 

vaccines

 tend to have a relatively low effectiveness compared with other vaccinations. Remember in The big picture, where you learned that flu vaccines need to be updated every year and remain only around around 40–60% effective? It is important to note, however, that the flu vaccine still plays an important role in preventing the spread and reducing the severity of influenza infections. Vaccines are covered in more detail in section C3.2.16.

Influenza can carry out a process known as reassortment. This occurs when an influenza virus infects a host cell that is already infected with a different strain of influenza, and the two viruses exchange genetic material, leading to the emergence of new strains that may have new or different characteristics. Reassortment is thought to be responsible for many of the global influenza pandemics.

Case study: rapid evolution in SARS-CoV-2

COVID-19 is an infectious disease caused by the SARS-CoV-2 virus (Figure 1), which, like HIV and influenza, is an RNA virus, and is therefore more prone to mutations than DNA viruses. The spike proteins on the surface of the virus give the coronavirus their name, ‘crown-like’, and play a key role in recognition of the host cell and fusion with the host cell membrane. Although some mutations will have a deleterious effect (harmful or damaging to the virus) or no effect, some mutations in the sequence of these spike proteins have been shown to increase transmissibility, reduce vaccine efficacy and aid the virus in evading the host cell immune system.

As with HIV and influenza, SARS-CoV-2 is considered to have a high replication rate, meaning it makes many copies of itself inside a host cell in a set period of time, increasing the likelihood that mutations will occur and accumulate.

When genetic recombination and mutations accumulate that make a virus sufficiently different to the original virus, these new viruses are known as ‘variants’, such as the SARS-CoV-2 Delta and Omicron variants. Because changes in the RNA sequence of SARS-CoV-2 are so frequent, they can be used to track the transmission of the virus, which can be helpful in developing public health measures to limit the spread of the disease.