21.3 Prevention and Treatment of Viral Infections

Prevention and Treatment of Viral Infections

Viral infections pose a major challenge to human health because viruses hijack host cells to replicate, making them difficult to target without harming the host. Prevention and treatment strategies range from vaccines that train the immune system before infection occurs to antiviral drugs that disrupt viral replication during an active infection. A key complication is that viruses mutate rapidly, which can reduce the effectiveness of both vaccines and drugs over time.

Prevention Mechanisms of Vaccines

Vaccines work by introducing viral antigens (proteins or other molecular components) to the immune system before you encounter the actual virus. This primes your body to respond quickly if the real virus shows up later.

Here's what happens after vaccination:

1. Viral antigens are presented to the immune system
2. B cells are activated and produce antibodies specific to those antigens
3. Memory B cells and memory T cells form, providing long-lasting immunity so your body can mount a faster, stronger response upon future exposure

Types of vaccines:

• Live attenuated vaccines contain weakened versions of the virus that can still replicate but don't cause disease. Because the virus replicates, these vaccines tend to produce strong, long-lasting immune responses. Examples include the measles, mumps, rubella (MMR) vaccine and the varicella (chickenpox) vaccine. The tradeoff is that they can't be given to immunocompromised individuals.
• Inactivated vaccines contain killed viruses that cannot replicate. They're safer for immunocompromised patients but typically require multiple doses (boosters) and may provide shorter-lasting immunity. Examples include polio (IPV) and hepatitis A vaccines.
• Subunit vaccines contain only specific viral proteins or antigens rather than the whole virus. They're safer than live attenuated vaccines but often need adjuvants (substances that enhance the immune response) to be effective. Examples include the hepatitis B and human papillomavirus (HPV) vaccines.
• Nucleic acid vaccines (mRNA or DNA) deliver genetic material that encodes a viral antigen. Your own cells then produce that antigen, which triggers an immune response. The Pfizer-BioNTech and Moderna COVID-19 vaccines are mRNA vaccines.
• Viral vector vaccines use a modified, harmless virus (the "vector") to deliver genetic material encoding the target virus's antigens into your cells. The Johnson & Johnson COVID-19 vaccine is an example.

Antiviral Drugs vs. Vaccines

These two tools serve fundamentally different purposes. Vaccines are preventive: they prepare your immune system before infection. Antiviral drugs are mainly therapeutic: they treat active infections by targeting specific stages of the viral life cycle.

Antiviral drugs work by interfering with viral replication at specific steps:

1. Entry inhibitors block the virus from attaching to or entering host cells
2. Nucleoside/nucleotide analogues mimic the building blocks of nucleic acids, interfering with viral genome replication when the viral polymerase incorporates them
3. Protease inhibitors block the enzymes that cleave viral polyproteins into functional pieces, preventing viral protein maturation
4. Integrase inhibitors prevent the viral genome from integrating into the host cell's DNA (used specifically for retroviruses like HIV)

Antivirals can reduce the severity and duration of illness. Examples include oseltamivir (Tamiflu) for influenza, remdesivir for COVID-19, and combination antiretroviral therapy (ART) for HIV. Some antivirals can also be used as prophylaxis, meaning they're given to high-risk individuals before or shortly after exposure to prevent infection.

Vaccines, by contrast:

• Do not directly target the viral life cycle
• Prepare the immune system to respond rapidly upon encountering the virus
• Provide long-term protection, sometimes lasting years or a lifetime
• Contribute to herd immunity when enough of the population is vaccinated, which protects individuals who can't be vaccinated (such as immunocompromised people)

Challenges in Treating Mutating Viruses

Viruses, especially RNA viruses, have high mutation rates because their polymerases lack proofreading ability. This generates genetic diversity and creates several problems for prevention and treatment.

Antigenic drift is the gradual accumulation of point mutations in genes encoding viral surface antigens. Over time, these small changes can make the virus look different enough that existing antibodies (from vaccination or prior infection) no longer recognize it well. This is why influenza vaccines need to be updated annually to match the predicted dominant circulating strains.

Antigenic shift is a more dramatic change that occurs specifically in influenza viruses. It happens when two different influenza strains or subtypes infect the same cell and exchange genome segments through genetic reassortment. This can produce a novel virus with surface antigens the human population has never encountered, potentially triggering a pandemic. The 2009 H1N1 pandemic arose through this mechanism.

Drug resistance develops when mutations in viral enzymes (the targets of antiviral drugs) change the enzyme's shape enough that the drug no longer binds effectively. This is a major concern with HIV treatment, which is why HIV is treated with combination therapy using multiple drugs with different mechanisms of action. Using several drugs simultaneously makes it far less likely that a single mutation will confer resistance to all of them.

Viral latency and reservoirs present another challenge. Some viruses, like HIV and herpesviruses, can enter a latent state in certain cell types, meaning they integrate into the host genome or persist without actively replicating. In this state, the virus is essentially invisible to both the immune system and antiviral drugs. These viral reservoirs can reactivate later, causing recurrent infections and making complete eradication of the virus from the body extremely difficult.

To address these challenges, researchers are pursuing several strategies:

• Combination therapies using multiple antiviral drugs with different targets
• Broadly neutralizing antibodies that target conserved (less mutation-prone) regions of viral antigens
• Novel approaches such as gene editing technologies to target latent viral reservoirs

Additional Considerations in Viral Infections

• Viral tropism refers to which cell types, tissues, or organs a virus preferentially infects. Tropism is determined by the match between viral surface proteins and host cell receptors. It directly influences disease symptoms and shapes treatment strategies.
• Zoonotic viruses are those that jump from animal hosts to humans (e.g., SARS-CoV-2, Ebola, influenza A). Because humans have limited pre-existing immunity to these viruses, zoonotic spillover events can lead to outbreaks that are difficult to contain.
• Interferons are signaling proteins naturally produced by virus-infected cells. They alert neighboring cells to activate antiviral defenses. Synthetic interferons can be used therapeutically to boost the immune response against certain viral infections (e.g., interferon-alpha for hepatitis B and C).
• Monoclonal antibodies are lab-engineered antibodies designed to bind specific viral proteins. They can be used as treatments for active infections or as short-term prophylaxis, though their effectiveness can be reduced by viral mutations in the targeted protein.