Can Cats Still Get Calicivirus After Vaccination? The Surprising Answer


Feline calicivirus (FCV) is a common viral infection in cats that causes oral ulcers, upper respiratory issues like sneezing and nasal discharge, and sometimes lameness. The symptoms are often mild but can become severe in some cases. FCV is easily spread between cats through direct contact or exposure to contaminated surfaces.

While vaccines for FCV are available as part of routine cat vaccinations, questions remain about their effectiveness in fully preventing infection. It’s not uncommon for cats to develop FCV infections even after vaccination. However, vaccines may still reduce disease severity and transmission.

What is Feline Calicivirus?

Feline calicivirus (FCV) is a highly contagious virus that causes respiratory illness in cats. It is one of the most common upper respiratory infections in cats. The virus spreads through infected saliva, nasal secretions, and eye discharge. Cats usually get infected through direct contact with an infected cat, sharing food bowls and litter boxes, or exposure to contaminated surfaces.

Common symptoms of FCV include:

  • Oral ulcers
  • Upper respiratory illness (sneezing, nasal discharge)
  • Fever
  • Drooling
  • Lethargy
  • Loss of appetite
  • Lameness or limping

In severe cases, FCV can lead to pneumonia, mouth and tongue ulcers, and chronic limping due to painful inflammation of joints and tissues of the feet. While most cats recover, FCV symptoms can persist or recur periodically.

Available Vaccines

There are several vaccines available on the market that protect against feline calicivirus (FCV) infection, often in combination with other common feline viruses like feline herpesvirus and feline panleukopenia virus (feline distemper). These include vaccines like PureVax, Fel-O-Vax, and Nobivac.

The most common FCV vaccines are modified live vaccines (MLV) that contain weakened forms of live FCV. When administered, these weakened viruses trigger an immune response that generates antibodies against FCV without causing illness in the vaccinated cat. This provides protection against future infection from exposure to fully virulent strains of the virus (Cornell).

Some combination vaccines also include chlamydia and leukemia components. Annual booster vaccination is recommended as immunity can decrease over time. Maternal antibodies passed to kittens can interfere with vaccination, so a series of vaccines starting as early as 6-8 weeks is required for protection (Animal Emergency Care).

Are the Vaccines Effective?

Studies have shown that readily available modified live and inactivated vaccines can provide good protection against calicivirus in cats. One study testing an inactivated vaccine found it provided 52% efficacy in reducing disease signs after challenge with virulent feline calicivirus (FCV) strains [1]. Another inactivated vaccine reduced median clinical scores by 82.6% compared to unvaccinated cats when challenged with FCV [2].

However, there are some factors that can reduce vaccine efficacy. Not all vaccines protect equally well against the many diverse strains of FCV in circulation. Mutations in field strains can allow FCV to evade some immune responses generated by current vaccines [2]. More research is still needed to improve and update vaccines against emerging FCV strains.

Breakthrough Infections

Even in vaccinated cats, breakthrough infections with feline calicivirus can still occur. While the vaccines provide good protection overall, they do not completely prevent infection in all cats.

There are a few potential reasons why a vaccinated cat might still get infected:

  • Emergence of new calicivirus strains that are not well covered by current vaccines. As the virus evolves, antigenic differences can develop that allow it to evade existing immunity (Dawson et al., 1993).
  • Low or incomplete immune response in some cats, leaving them susceptible despite vaccination. Factors like age, health status, and genetics can affect an individual’s response (Spiri et al., 2021).
  • Waning vaccine immunity over time. Protection may decrease the longer it has been since the last vaccine booster (Dawson et al., 1993).

In these situations, the cat may mount a partial immune response and experience milder symptoms, but still become infected and shed virus. Breakthrough cases tend to be less severe than in unvaccinated cats.

Managing Calicivirus in Vaccinated Cats

Even after vaccination, it’s important to monitor your cat’s health for any signs of calicivirus infection. The American Association of Feline Practitioners (AAFP) recommends an annual veterinary examination to assess vaccine coverage and overall health (AAFP Vaccination Guidelines).

According to Cornell University’s Baker Institute, breakthrough infections can occur in vaccinated cats due to insufficient immune response or exposure to new viral mutants that evade immunity ( Seek prompt veterinary care if you notice symptoms like fever, oral ulcers, limping, or nasal discharge.

Booster vaccinations may be recommended if your cat has ongoing exposure risk. The AAFP states that in high risk situations, boosters every 1-2 years can improve protection. Your veterinarian can determine if more frequent boosters are needed based on your cat’s lifestyle and health status (AAFP Vaccination Guidelines).

Mitigation Strategies

There are several ways cat owners can help limit their cat’s exposure to calicivirus and reduce the chances of infection:

  • Keep cats indoors and avoid contact with stray or unvaccinated cats. Calicivirus is very contagious so limiting exposure is key.
  • Clean and disinfect food bowls, litter boxes, toys, bedding etc. regularly. Bleach solutions can kill the virus on surfaces.
  • Wash hands before and after interacting with your cat to prevent spreading the virus.
  • Isolate any new cats or cats showing signs of illness to prevent disease transmission.
  • Use separate food bowls, litter boxes and bedding for multiple cats to limit viral spread.
  • Discuss disinfectants safe for cats with your veterinarian.

Proper hygiene and limiting exposure are the best ways to prevent calicivirus infection in vaccinated cats. Keeping the environment clean and virus-free can help stop the spread of infection.


The prognosis for breakthrough calicivirus infections in vaccinated cats is generally good, especially if treated promptly. According to the Cornell Feline Health Center, most cats recover within 1-2 weeks with supportive care such as fluids, anti-inflammatories, and antibiotics for secondary infections (source). However, the virus can cause permanent damage to teeth and bones in some cases.

While vaccination provides robust protection, it does not completely prevent infection. Cats can become reinfected later in life, though subsequent infections tend to be milder. Booster vaccines are important to maintain immunity as the cat ages (source). In multi-cat households, isolation of infected cats is recommended to avoid spread to housemates.

Latest Research

Recent studies have made promising advances in feline calicivirus vaccines and diagnostics. An experimental vaccine containing a chimeric FCV with the capsid of the virulent systemic strain FCV-Ari and the nonstructural proteins of the F9 vaccine strain provided broad cross-protection in cats while limiting virulence and vaccine-induced oral lesions (1).

Researchers have also developed RT-qPCR assays that can differentiate vaccine and virulent field strains by targeting specific genomic regions, allowing better diagnostics and epidemiological tracking (2). Work is ongoing to further characterize the diversity of FCV strains circulating globally using full genome sequencing and phylogenetic analysis (3).

Overall, the latest research is focused on improving vaccine efficacy against heterologous strains, developing safer and more targeted vaccines, and enhancing diagnostic capabilities to identify emergence of new virulent variants.


As we’ve discussed, it is possible for a vaccinated cat to still contract calicivirus. This is because no vaccine provides 100% protection against disease. However, vaccination remains the best defense in preventing severe calicivirus infections in cats. Outbreaks may still occur in multi-cat environments, but symptoms are often milder in vaccinated individuals.

For cat owners, prevention starts with keeping up to date on core vaccines like feline viral rhinotracheitis, calicivirus, and panleukopenia. Reducing exposure risk by limiting interactions with unknown cats is also key. Within the home, measures like cleaning litter boxes daily, washing hands routinely, and separating sick cats can help stop the spread of infection.

If a vaccinated cat does develop signs of calicivirus, owners should isolate the cat, call the veterinarian, and disinfect any contaminated items or areas. Supportive care like nutritional support and pain management may be recommended. While some cats fully recover, others may experience chronic oral inflammation or permanent tooth loss.

By understanding that breakthrough cases can occur, and taking steps to lower risks, cat owners can continue to protect their feline companions against this prevalent virus.

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