Mice were essential during the Covid-19 pandemic, enabling the rapid development of effective vaccines and treatments. Humanised mice expressing the human ACE2 receptor were used to understand SARS-Cov-2 entry, immune response and vaccine effectiveness.  
Years before the Covid-19 pandemic, mice had already been used to gain important insights about other related diseases, such as SARS, which caused deadly widespread outbreaks in the early 2000s, and MERS, which first emerged in 2012. It was thanks to these earlier studies that researchers could study how the SARS-CoV-2 virus infected human cells, without needing to start from scratch with a new mouse model that would have inevitably slowed the pace of Covid-19 research.   
Humanised mice for Covid-19 allow researchers to answer key questions about the virus itself, such as how it begins to infect organisms and is transmitted from one body to another, how an infected host’s immune response reacts, as well as the short- and long-term effect of various treatments and experimental vaccines. All of these aspects are needed to shed light on how the disease works in order to develop effective drugs and measures against it.  

Mice are also used to test innovative vaccine strategies for viruses that mutate rapidly, such as HIV. Because HIV changes so quickly, designing vaccines that trigger broad protective immune responses is extremely challenging. Recent studies used mice to test approaches that ‘train’ the immune cells to recognise a wide range of HIV strains. The immune cells were taught to bind to different vulnerable parts of the virus, blocking multiple HIV types from infecting mice. 

Studies in mice are central to flu research, contributing to the understanding of why some influenza strains become more dangerous, how immunity develops and how to design better vaccines.  
Researchers from the University of Cambridge and the University of Glasgow showed that avian flu viruses can survive fever-like conditions in mice, unlike human-origin flu strains. This resilience was linked to the PB1 gene, which may also spread when bird and human flu viruses infect the same host, potentially generating more harmful variants.  
Researchers have long been working to develop a universal flu vaccine. A study conducted at the University of Gothenburg, in Sweden, showed that a small alpaca-derived antibody protected mice against multiple influenza types.  

A major advance is the first approved treatment for breast cancer – the monoclonal antibody, Herceptin – which developed due to initial research in mice. These studies laid the foundations for the discovery and understanding of the HER2 protein, which can be targeted to reduce tumour growth. Translated into the clinic, studies of breast cancer patients have shown that adding Herceptin to chemotherapy improves survival and keeps cancer at bay for longer compared to chemotherapy alone. 

Studies in mice, conducted at McMaster University in Canada, revealed that the protein BACE1 helps lung cancer cells spread to the brain. Blocking the protein BACE1 in mice with the drug Verubecestat, which was originally developed for Alzheimer’s, reduced the spread of lung cancer to the brain and extended mice’s survival. 

Mouse studies at the École Polytechnique Fédérale de Lausanne in Switzerland tracked how lung tumours adapt to therapy getting more energy from surrounding cells, revealing strategies to prevent treatment resistance.  

Research in mice contributed to the development of a type of immunotherapy cancer treatment, which uses the body’s immune system to attack and kill cancer cells (called immune checkpoint inhibitors). This work was recognised by the 2018 Nobel Prize in Physiology or Medicine.  

Mice can provide important insights into the mechanisms of Alzheimer’s, the most common type of dementia, for example by showing hallmarks of the disease, such as the loss of neurones in the brain, or symptoms like memory loss. 
GA mice can be engineered to develop key features of Alzheimer’s, including protein buildup in the brain and progressive memory decline. These GA animals allow researchers to track how the disease begins long before symptoms appear, something that is impossible in humans, namely understanding how beta-amyloid accumulates in Alzheimer's disease and its relationship to pathology and cognitive decline. 
In what marks the cusp of first-generation treatments for Alzheimer's disease, a monoclonal antibody treatment, with mouse antibodies, modifies the disease itself, showing success in a clinical trial by slowing brain decline in Alzheimer's patients.  

For Parkinson’s disease, strategies to protect against its progression or symptoms have been underpinned by animal studies. A study at the University of Geneva, Switzerland, used both mice and fruit flies to identify a gene that can protect against the disease. 

Huntington's disease is an inherited neurodegenerative disease that leads to cognitive and motor impairment. Recently, a groundbreaking therapy for Huntington's disease was reported to slow disease progression by 75% in human clinical trials after years of studies in mice and other mammals. 

Mice have been used to study depressive behaviours and symptoms, which in turn can help to identify targets in the brain for drugs that reduce the effects of depression. For example, research in mice and cells at EARA member the University of Helsinki, Finland, found that the psychedelic drugs LSD and psilocin can be used as antidepressants without triggering hallucinations. Scientists from the University of Hong Kong and Guangdong University of Chinese Medicine, China, used eye stimulation as a successful treatment for depression in mice. 
Another study at the MPI of Neurobiology (now the MPI for Biological Intelligence), Germany, looked at mice to understand the body-brain interactions on the regulation of emotion and fear. Research at University College of Cork, Ireland, has investigated how signals between gut bacteria and the brain can influence the behaviour and emotions of mice, which could help develop the understanding of anxiety and depression in people. 
A team at the University of Colorado Boulder, US, identified a previously unknown brain circuit in a specific zone of the midbrain that controls how mice respond and adapt to sudden threats, like a predator’s approach, using real-time brain imaging and optogenetics – a technique combining light and genetic engineering to control specific neurones. These type of studies are essential to understand anxiety and other stress-related conditions.