Types of vaccine

Inactivated adjuvanted vaccines against RHDV1

Due to the devastating effects of RHD in China, a vaccine was quickly developed. It was produced from inactivated virus obtained from the liver and spleen of infected rabbits. Live rabbits are needed to cultivate the RHD calicivirus because it cannot be grown in tissue culture or embryonated chicken’s eggs. After they were developed in China, inactivated adjuvanted vaccines against RHD (e.g. Cylap and Lapinject) became available in other countries. These vaccines protected rabbits against RHDV1 although they had potential to cause tissue reactions at the inoculation site. They also posed a risk to the person administering the vaccine if accidental self-inoculation occurred, not because of the virus but because of a reaction to the adjuvant in the vaccine.

Recombinant vaccine against myxomatosis and RHDV1

In 2012, a bivalent vaccine against both myxomatosis and RHD came on the market. (Nobivac Myxo-RHD, MSD Animal Health). This vaccine was manufactured differently from previous vaccines against RHD. It is a recombinant vector vaccine that is made by inserting the RHDV capsid gene into an attenuated myxomatosis virus so immunity to both myxomatosis and RHD is stimulated (Spibey et al. 2012). The technology used to produce this vaccine allows the RHD component to be produced in vitro instead of using live rabbits for virus cultivation. In the UK and other countries, Nobivac Myxo-RHD quickly superseded previous myxomatosis and RHD vaccines because it protected against both diseases with a single annual injection.

Inactivated adjuvanted vaccines against RHDV2

 In 2014, RHDV2 spread into UK. By 2016, it had reached epidemic proportions (Harcourt-Brown et al. 2020a) and it was becoming obvious that Nobivac Myxo-RHD was not preventing rabbits from dying with RHD (Harcourt-Brown et al. 2020b). Inactivated adjuvanted vaccines against RHDV2 had been developed in other countries where rabbit meat is popular and were imported into UK. Two of these vaccines subsequently obtained product licences in UK: Filavac VHD K C+V (Ceva Animal Health) and Eravac (Hipra Laboratories). Filavac is active against both RHDV1 and RHDV2. Eravac is active against RHDV2.

Recombinant vaccine against myxomatosis, RHDV1 and RHDV2

In May 2020, Nobivac Myxo-RHD PLUS, (MSD Animal Health) became available in UK and Europe. This vaccine is manufactured from two recombinant myxomavirus vectors each containing capsid protein genes from either RHDV1 or RHDV2. This triple vaccine offers protection against myxomatosis, RHD1 and RHD2 with a single annual injection. Unfortunately, switching to Nobivac Myxo-RHD PLUS in a rabbit that has previously been vaccinated with Nobivac Myxo-RHD is not straightforward due to vector interference.

Vector interference

Like all vaccines, initial exposure to an attenuated or inactivated virus triggers an immune response that primes the immune system by stimulating memory lymphocyte production. Subsequent exposure to the virus, either from booster vaccination or natural exposure, stimulates the memory lymphocytes leading to rapid production of antibodies.

In a rabbit that has been vaccinated with Nobivac Myxo-RHD, the immune system is primed against myxomavirus and RHDV1 but not primed against RHDV2. If this rabbit is subsequently given Nobivac Myxo-RHD PLUS, the rapid immune response to the myxomavirus vector can interfere with virus replication and lead to an inadequate immune response to RHDV2 antigen that the vectored myxomavirus contains. This phenomenon is known as ‘vector interference’ and hinders protective immunity to RHDV2 in rabbits that are switched from Nobivac Myxo-RHD to Nobivac Myxo-RHD PLUS. Immunity from natural exposure to myxomatosis can also result in vector interference.

Vector interference should be overcome if the immune system is already primed against RHDV2 by vaccination with an inactivated adjuvanted RHDV2 vaccine, such as Eravac or Filavac. The memory lymphocytes should respond rapidly when the rabbit is given Nobivac Myxo-RHD PLUS next time the rabbit is vaccinated.

Summary of vaccines available in UK 

Factors to consider when choosing a vaccine

There are several considerations when choosing which vaccine to use.

• Nobivac Myxo-RHD PLUS is a single annual vaccine against myxomatosis, RHDV1 and RHDV2. 
• In the face of an outbreak of RHD, the interval between administering a vaccine and the onset of immunity is important. Inactivated adjuvanted vaccines (Filavac, Eravac) take 7 days to become effective in comparison with Nobivac Myxo-RHD PLUS, which takes 3 weeks.
• Although RHDV2 has superseded RHDV1 in many parts of the world, vaccination against both RHDV1 and RHDV2 is recommended for pet rabbits because RHDV1 may be present in the wild rabbit population and is still a threat.
• Some owners have ethical objections to inactivated adjuvanted vaccines because rabbits are used to produce the vaccine.
• Vector interference is a problem in rabbits that have had a previous vaccination against myxomatosis but not RHD. Switching from the old Nobivac Myxo-RHD to Nobivac Myxo-RHD PLUS is not straightforward.
• Vector interference can occur in pet rabbits that have contact with wild rabbits and/or flying insects during an outbreak of myxomatosis. There is no interference between myxomatosis and RHD vaccination with inactivated adjuvanted vaccines.
• Choosing a vaccine is complicated in rabbits with lapsed vaccinations. For examples, vector interference could wane with time in rabbits that had Nobivac Myxo-RHD some years ago. Immunity to Filavac lasts for more than a year so it is possible for rabbits with lapsed vaccines to be protected by a single dose of Nobivac Myxo-RHD PLUS. If there is any doubt about which vaccines to use, the safest option is to administer an inactivated adjuvanted vaccine (Filavac or Eravac) followed by Nobivac Myxo-RHD PLUS two weeks later
• Inactivated adjuvanted vaccines were effective in protecting pet rabbits during a naturally occurring outbreak of RHDV2 in UK (Harcourt-Brown et al. 2020)  although there were some failures.

Vaccinating baby rabbits against RHDV2

Baby rabbits are susceptible to RHDV2 and it is important to protect them. Maternal immunity can interfere with the response to RHDV2 vaccination so the age at which baby rabbits can be vaccinated varies with both the brand of vaccine and the presence of maternal immunity in mothers that have been vaccinated. Rabbits born to vaccinated mothers should be over 7 weeks of age before receiving Nobivac Myxo-RHD Plus because of maternal immunity.

Rabbits born from unvaccinated mothers will have no maternal immunity to RHDV2 and can be vaccinated as early as 4 weeks of age with Filavac (personal communication), 30 days with Eravac and 5 weeks with Nobivac Myxo-RHD PLUS. The onset of immunity is longer for Nobivac Myxo-RHD Plus (3 weeks) than for Filavac and Eravac (7 days). This may be an important factor in the face of an outbreak. It is recommended that rabbits under 10 weeks of age that are vaccinated with Filavac should have second vaccination after 10 weeks to ensure immunity lasts for a year.

Some reasons for vaccine failure

Although it is easy to blame a vaccine when a vaccinated rabbit dies from RHD, there are several possible explanations for vaccine failure. Many of these are overcome if the rabbit is vaccinated by a veterinary surgeon or veterinary nurse after a health check and vector interference is taken into consideration.

• Already incubating the disease: Rabbits are often vaccinated against RHDV2 when other rabbits in the household have died suddenly. If the cause of death was RHDV2, then all the rabbits in the household will have been exposed and could be incubating the disease at the time of vaccination. This was identified as the main cause of inactivated adjuvanted vaccine failure in a study by Harcourt-Brown et al. 2020
• Transport conditions: Vaccines need to be kept at the correct temperature. They are refrigerated at a specific temperature during shipping from the manufacturers to the wholesalers and subsequent transport to veterinary practices. Vaccines that are not transported at the correct temperature may not be as effective.
• Incorrect storage of vaccine: Veterinary surgeries have to comply with stringent regulations about the storage of vaccines. In order to ensure that vaccines are effective, they must be kept refrigerated at the correct temperature and practices keep records of maximum and minimum temperature measurements in all fridges that contain vaccines. These requirements are not met by owners or breeders that store vaccine in domestic fridges.  
• Incorrect administration: Rabbit skin is thin. It is easy for a needle to penetrate two layers of skin that is tented up to insert a needle into. This means the vaccine is not injected into the subcutaneous space but is squirted on to the skin instead. This is less to happen if a veterinary surgeon or veterinary nurse administers the vaccine. If it does happen, another vaccine needs to be administered.
• Using up multidose vials: Vaccines against RHD are available as multidose vials. These are intended for use within hours once the vial is opened or the vaccine mixed with diluent. Vaccines are not cheap and there is a temptation to store opened vials and use them later. This will affect the efficacy of the vaccine. Vaccines administered at a veterinary practice are generally from single dose vials unless vaccination clinics are organised in which multidose vials are used. Unused vaccine from these clinics is disposed of according to the regulations covering the proper disposal of medication.
• Underlying disease: The vaccine datasheets say that only healthy rabbits should be vaccinated. This is not because rabbits with underlying disease are at risk from the vaccine, but because the vaccine is less likely to be fully effective. Rabbits with underlying health problems may be immunologically challenged so they do not respond fully to a vaccination. Rabbits hide disease so it can be difficult to know if subclinical disease is present.
• Already incubating the disease: Rabbits are often vaccinated against RHDV2 when other rabbits in the household have died suddenly. If the cause of death was RHDV2, then all the rabbits in the household will have been exposed and could be incubating the disease at the time of vaccination.
• The rabbit was not vaccinated in the first place: This scenario is unlikely in pet rabbits. Rabbits that have been vaccinated should come with a vaccination certificate with the name, date and batch number of the vaccine they were given. If this is not available, the safest option is to administer another vaccine
• Individual variation: No vaccine is 100% effective. There are always individuals that do not respond to vaccination.

For commercial rabbits where hundreds of rabbits are vaccinated, it is possible to miss some individuals, or they may not have responded properly to the vaccine because of underlying disease or problems with administration. This was a reason for the recommendation for 6 monthly intervals between vaccination in meat rabbits by some vaccine manufacturers. This recommendation does not apply to rabbits kept as pets.