Adenoviruses are a family of viruses that can cause a wide variety of illnesses in humans, from the common cold to gastrointestinal infections to pink eye. And recently, scientists have used these viruses as the basis for several COVID-19 vaccines, including those produced by Johnson & Johnson and AstraZeneca.
What is an adenovirus?
There are 88 types of adenovirus known to infect humans, and these are grouped into seven different species, A through G, according to a 2019 report in the journal Scientific Reports. The viruses circulate year-round, meaning they don't show strong seasonality like influenza viruses, for instance. Additional adenoviruses infect a broad range of vertebrate animals, including mammals, birds, reptiles and even the occasional fish, according to a 2019 report in the journal FEBS Letters.
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In humans, adenovirus infections most commonly cause mild respiratory symptoms, but sometimes, the viruses can cause severe disease; people with compromised immune systems, existing respiratory diseases or heart diseases face a higher risk of severe infection than others, according to the Centers for Disease Control and Prevention (CDC).
No publicly available vaccines protect against adenovirus infection, although one existing vaccine is sometimes given to U.S. military personnel, according to the CDC. (This vaccine works against type 4 adenoviruses, classified as species E, and type 7 adenoviruses, which fall under species B.)
That said, scientists have used modified adenoviruses to build other vaccines, such as those that protect against COVID-19. These modified viruses can no longer infect human cells, but they instead act as a vehicle for the vaccine to enter the body.
Adenovirus infections can often cause common cold or flu-like symptoms, fever and sore throat, according to the CDC. They can also cause pink eye, or conjunctivitis; inflammation in the airways of the lungs, called acute bronchitis; pneumonia, an infection of the lungs; and inflammation of the stomach or intestines, known as acute gastroenteritis. The gastrointestinal infections result in diarrhea, vomiting, nausea and stomach pain.
Less commonly, adenoviruses can cause urinary tract infection, bladder inflammation or infection and even neurologic diseases that affect the spinal cord and brain.
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Adenoviruses spread between people through close personal contact, as well as through the air when an infected person coughs or sneezes. People can also pick up the virus from contaminated surfaces and become infected by touching their mouth, eyes or nose before washing their hands. Some strains of adenovirus can be spread through an infected person's stool, and very rarely, the viruses can spread through water, such as in swimming pools. (Outbreaks can be avoided with adequate levels of chlorine in pools, the CDC states.)
You can reduce the spread of adenoviruses by washing your hands frequently with soap and water; not touching your face with unwashed hands; and avoiding close contact with sick people. If you are sick, you can help prevent spread by staying home, washing your hands often and coughing and sneezing into tissues or your upper arm, rather than your hands. And infected person should also avoid close contact with others and make sure not to share utensils or cups with other people.
How is adenovirus diagnosed and treated?
Adenovirus infections can be diagnosed using clinical tests that detect the virus in samples from the patient, or detect specific substances that the virus produces, called antigens, according to the CDC. These diagnostic methods may include antigen tests; polymerase chain reaction (PCR) tests, which detect viral genetic material; virus isolation, where samples are cultured in a lab dish; and serology, which uses blood samples.
Some people can "shed" adenovirus particles for weeks after their infection has cleared; that means sometimes, even if someone tests positive for adenovirus, it may not be the cause of the person's current symptoms. Doctors may perform additional tests to rule out other diseases, according to the CDC.
There are no specific treatments for adenovirus infections, and most cause mild symptoms and don't require treatment at all. When treatment is needed, doctors prescribe medicine to relieve symptoms. The antiviral cidofovir has occasionally been used to treat severe adenovirus infections in people with weak immune systems, but it's not specifically approved for the purpose, according to the CDC.
Do adenovirus vaccines exist?
Outbreaks of adenovirus in the military led the U.S. Department of Defense to begin vaccinating military recruits against two strains of the virus in 1971, according to Medscape.
When vaccine production stopped in 1996, due to economical reasons, cases of adenovirus in the military increased, as the viruses spread easily in close quarters. This re-emergence of adenovirus led to the reintroduction of the vaccine among recruits in 2011, Medscape reported. The vaccine prevents an estimated 15,000 cases of adenovirus infections in U.S. military recruits, according to the U.S. Army Medical Materiel Development Activity.
A recent study, published in 2018 in the journal Emerging Infectious Diseases, looked at adenovirus respiratory infections in nonmilitary individuals and concluded that the vaccine should also be considered for susceptible groups outside the military, such as those living in long-term care facilities or college dorms.
Dr. Amesh Adalja, a senior scholar at the Johns Hopkins Center for Health Security, previously told Live Science that he agreed with this conclusion that, "because [adenovirus] does cause a considerable burden of illness, we want to explore" the ability to use the vaccine outside of the military context.
For example, the vaccine may benefit people at high risk of contracting these viruses, such as patients with lung disease and others with compromised immune systems, but it may even benefit the general population, given that people in congregate living situations are prone to infection, Adalja said. However, future studies would be needed to examine which segments of the population would benefit most, and whether vaccination would be cost-effective, he said.
How do vaccines made from adenoviruses work?
Adenoviruses can be used as so-called viral vectors in vaccines, meaning they carry the ingredients of a given vaccine into cells.
For example, the COVID-19 vaccine made by AstraZeneca and the University of Oxford contains a weakened adenovirus that naturally infects chimpanzees. Scientists modified the virus so it cannot replicate in human cells, and then they added genes that code for the coronavirus spike protein. Inside the body, the vaccine enters cells and delivers these spike protein genes to the nucleus; the cells then use the delivered genes to build the spike protein itself. The spike proteins trigger an immune response, training the body to recognize and attack the coronavirus SARS-CoV-2 if a person encounters it.
Similarly, the COVID-19 vaccine developed by Johnson & Johnson's Janssen contains a human adenovirus called Ad26, which has been modified so it cannot replicate and instead carries coronavirus genes. The company used the same method to create its approved Ebola vaccine as well as vaccines for other diseases that are still in clinical trials.
Recently, scientists are finding that the AstraZeneca shot seems to cause a very rare autoimmune response that causes an unusual type of blood clot. The Johnson & Johnson shot has also been tied to a similar type of blood clot, though experts don't yet know if the vaccine is causing them. It's not clear in either case whether the adenovirus vector has anything to do with the rare side effect.
Importantly, adenoviruses contain double-stranded DNA, and the coronavirus genes are placed within this structure, The New York Times reported. The COVID-19 vaccines made by Pfizer and Moderna also deliver coronavirus genetic material into the body, but in the form of messenger RNA (mRNA), a much less stable molecule. The double-stranded DNA in the adenovirus-based vaccines makes them robust enough to be stored at warmer temperatures than the mRNA-based vaccines; additionally, the outer coat of an adenovirus is sturdier than the protective fatty coating made to surround mRNA in vaccines.
In addition to the AstraZeneca and Johnson & Johnson COVID-19 vaccines, the shot produced by CanSino Biologics and the Beijing Institute of Biotechnology also contains a weakened adenovirus, one that normally infects humans called Ad5. The so-called Sputnik V vaccine, created by the Russia Ministry of Health's Gamaleya Research Institute, contains two human adenoviruses.
Live Science reporter Rachael Rettner contributed material to this article.
Originally published on Live Science.