For many veterinary practitioners canine vaccination programs have been "practice management tools" rather than

medical procedures.  Thus, it is not surprising that attempts to change the vaccines and vaccination programs based

on scientific information have created great controversy and unique methods of resistance to the proposed changes

have been and are being developed.  For some practitioners the issues are not duration of immunity for the vaccines,

nor which vaccines are needed for the pet, instead it is felt that every licensed vaccine should be given to every pet

on an annual or more often basis. 

Ironically this is fostered by the fact that multivalent products with 7 or more vaccine components can be purchased

for the same price or less than a product with one or two vaccine components.  A "more is better" philosophy prevails

with regard to pet vaccines.  On many occasions practitioners say that "I know many of the vaccines I administer

probably aren’t needed but it won’t hurt to give them and who knows the animal may need them some time during

their life because of unknown risk."  I have also been told by many practitioners that "I believe the duration of

immunity for some vaccines like distemper, parvovirus and hepatitis is many years, but until I find another way to

get the client into my office on a regular basis I'm going to keep recommending vaccines annually." 

Annual vaccination has been and remains the single most important reason why most pet owners bring their pets for

an annual or more often "wellness visit."  The importance of these visits for the health of the pet is exceptional. 

Therefore, dog owners must understand the vaccines are not the reason why their dog needs an annual wellness visit. 

Another reason for the reluctance to change current vaccination programs is many practitioners really don’t

understand the principles of vaccinal immunity. 

A significant number of practitioners believe :

1

The annual revaccination recommendation on the vaccine label is evidence the product provides immunity

for (only) one year. – Not True

2

That they are legally required to vaccinate annually and if they don’t they will not be covered by AVMA

liability insurance if the animal develops a vaccine preventable disease - Not True.  Furthermore, certain

companies will not provide assistance if practitioners don’t vaccinate annually with core vaccines. 

Not True – In fact most of the companies have now demonstrated their core products provide at least

3 years of immunity.

3

That not revaccinating will cause the animal to become susceptible soon ( days or a few weeks )

after the one year. – Not True

4

If the animal is not revaccinated at or before one year the "whole vaccination program needs to be

started again". – Not True

5

If they don’t continue to revaccinate annually, diseases like canine distemper, canine parvovirus and

infectious canine hepatitis ( CAV-1 ) will "reappear and cause widespread disease similar to what was

seen prior to the development of vaccines for these diseases." – Not True

6

That if the revaccination "doesn’t help, it won’t hurt." – Not True

7

That giving a vaccine annually that has a duration of immunity of 3 or more years provides much better

immunity than if the product is given only once during the three years. – Not True 

In fact, there are regional/state rabies programs that suggest annual rabies vaccination programs provide

better protection than revaccination once every three years regardless of whether a 1 year or 3 year

rabies product is used. – Not True

8

That parvovirus vaccines only provide six months of immunity, thus they must give them semi-annually

and the CPV-2 vaccines need to be given with coronavirus vaccine to prevent enteritis. – Not True

9

"It’s much cheaper to revaccinate the pet annually than it is to treat the disease the animal will develop

because it didn’t get revaccinated annually."  The "better safe than sorry" philosophy -  It is less expensive

to prevent disease.  However, if the core vaccines are given as a puppy and again at a year of age,

then annual vaccination is not needed.  Furthermore, if a vaccine is given that is not needed and it

causes an adverse reaction that is unacceptable and very expensive.

10

They need to revaccinate all new dogs/cats coming to their clinic irrespective of vaccination history

even when vaccination records are available from another clinic.  Presumably the "other clinic" used the

wrong vaccine or didn’t know how to vaccinate. – Not True

11

”Dogs need to be revaccinated annually up to 5 to 7 years of age, then and only then would vaccination every three years be okay.” – Not True

12

"Surgical procedures, including anesthesia, are immunosuppressive thus dogs should be vaccinated prior

to or shortly after surgery." – Not True

13

"Because boarding kennels require annual or more often ( kennel cough every 3 to 6 months ) vaccination,

practitioners must continue vaccinating annually with all vaccines." – Not True – help change the kennel

rules through education and just use the vaccines that need to be given ( eg Kennel Cough..)

Note :

There are kennels that require every licensed vaccine and the vaccines must have been given within 1 year

or less prior to admission – help change these rules!  Those kennels that are members of the American Kennel

Association should be following the AAHA Guidelines, but many kennels do not belong to this association.

It will be necessary to correct many of these and additional misunderstandings by providing education to veterinary

practitioners, kennel owners and pet owners before significant changes in vaccination programs can or will occur

to reduce the over-vaccination of both cats and dogs. 

However it is equally important that we don’t, in our efforts to prevent over-vaccination :

1

fail to vaccinate often enough

2

fail to vaccinate all or as many pups with the core vaccines

3

fail to use products that are necessary

4

use products ( eg nosodes ) that don’t provide protection for our pets.

I believe every practitioner, kennel owner and dog owner should know the following general information about canine

vaccines and vaccination programs. 

What vaccines are needed for all puppies ? 

I do mean all pups, as we only vaccinate 50% of dogs.  If we could increase this percentage to 75%, we would be

able to eliminate many of the diseases prevented by core vaccines. 

The “core vaccines,” those that every pup should receive and identified as core by most canine vaccine experts

in the United States, include :

1

 Canine Parvovirus type 2 ( CPV-2 )

2

 Canine Distemper virus ( CDV )

3

 Canine Adenovirus type 2 ( CAV-2 )

4

 Rabies Virus ( RV )

When do the core vaccines need to be given ? 

As a minimum, puppies should be given at least one dose at 16 weeks of age or older.  Preferably, they should be

given three or more times starting at 6 to 9 weeks then at an interval of 2 to 4 weeks revaccinate 9 to 12 weeks

then again at 14 to 16 weeks.  It is critical that the last dose be given at 14 to 16 or more weeks of age. 

It is important not to give them earlier than 6 weeks unless there is a significant risk of a specific disease,

then give only the vaccine for the disease you want to prevent ( e.g. CPV-2 ). 

Never vaccinate a pup less than 4 weeks of age.  The most effective canine core products currently include modified

live and recombinant vaccines alone or in combination.  The combination products with CPV-2, CDV and CAV-2

currently often include canine parainfluenza ( CPI ) virus. 

New “core only” products have been and are being developed that don’t have CPI, however, the CPI will not cause

a problem if and when used as a parenteral 5 way combination product. 

After the puppy series is completed, revaccination is recommended again at one year of age or one year after the

last puppy vaccination ...  Rabies must be given again at 1 year, then every 3 years, whereas, the other core

vaccines need not be given again for at least 3 or more years.  There is no benefit from annual rabies vaccination

and most one year rabies products are similar or identical to the 3-year products with regard to duration of immunity

and effectiveness.  However, if they are 1 year rabies vaccines, they must be legally given annually ! 

Rabies vaccine is the only canine vaccine requiring a minimum duration of immunity study.  However, revaccination

annually does not necessarily improve immunity.  However, annual vaccination does significantly increase the risk

for an adverse reaction in the dog.  I would recommend, if you really want to be sure the puppy vaccination program

was successful, that a CDV and CPV-2 antibody titer be performed 2 or more weeks after the last puppy vaccination. 

Laboratory tests as well as “in-office test” for CDV and CPV-2 tests are available.  If there is no antibody, revaccinate

and perform a test two or more weeks after revaccination.  If you still don’t have antibody, change the product and

vaccinate again.  Antibody tests (titers) are very useful at these times to ensure the animal is immunized. 

The problem with antibody tests is they are very expensive, thus in general, these tests won’t be used. 

As an alternative to revaccinating at one year after completing puppy series for CDV, CPV-2 and CAV-2, I would

revaccinate at 6 months of age to ensure the animal has responded rather than waiting until 1 year of age. 

Then, revaccinate not more often than every 3 years.  The minimum duration of immunity for the core vaccines

except rabies is at least 7 years based on challenge and/or titers ( Table 1 ).  Thus revaccinating annually will not

improve protection.  Ironically "the better safe than sorry philosophy" can be equally applied to less vaccination,

since the animal that develops an adverse reaction (e.g. hives, facial edema, anaphylaxis) from a vaccine that

wasn’t needed is an example of "being sorry, not safe !"

What about all the other vaccines currently available for the dog ? 

They are non-core or optional vaccines that should only be given to animals that need them and only as often as

needed.  There are also some vaccines that are not recommended for any dogs.  The duration of immunity is not

known for certain non-core products, the efficacy is limited or not known and the risk vs. benefit factors are not

always well established nor understood.  The minimum duration of immunity for Leptospira vaccines is probably less

than one year, thus if required for a high risk dog, they may need to be given as often as semi-annually. 

Considering the low efficacy, the adverse event rate and the minimal risk for leptospirosis in many regions of the US,

certain practitioners are not using the current products.  However, if an animal is in a high-risk environment for

leptospirosis, the product to use should contain the 4 serovars ( there is no significant cross protection among the

4 current serovars ).  I recommend to start vaccination not earlier than 12 weeks of age, revaccinate in 2 to 4 weeks,

revaccinate at 6 months of age, revaccinate at a year of age and then you may have to revaccinate as often as every

6 to 9 months for optimal protection.  Using this program the animal should not develop clinical disease but it may

get infected and shed organisms in its urine.  Bordetella immunity may be less than one year and the efficacy for the

products is not well established.  Many animals receive "kennel cough" vaccines that include Bordetella and CPI with

or without CAV-2 every 6 to 9 months without evidence that this frequency of vaccination is necessary or beneficial. 

In contrast, other dogs are never vaccinated for kennel cough and disease is not seen.  CPI immunity lasts at least

3 years when given intranasally, and CAV-2 immunity lasts a minimum of 7 years when given parenterally for CAV-1,

but duration of immunity is probably less for CAV-2 ( eg 3 years ).  These two viruses in combination with Bordetella

bronchiseptica are the agents often associated with kennel cough, however, other factors play an important role in

disease ( e.g. stress, dust, humidity, molds, Streptococcal spp., mycoplasma, etc. ), thus kennel cough is not

a vaccine preventable disease because of the complex factors associated with this disease.  Furthermore, Kennel

Cough is often a mild to moderate self limiting disease.  I refer to it as the "Canine Cold."  My preference when a

kennel cough vaccine is used is the intranasal vaccine rather than the parenteral, but some dogs will not allow an

intranasal vaccine to be administered.

There is a new virus of dogs, an "equine-like influenza virus," that first infected greyhounds in Florida in 2004,

causing respiratory disease.  At present, it is not known whether canine influenza virus ( CIV ) will become an

important cause of canine respiratory disease, nor if it will be an emerging disease of dogs.  Recently, cases of CIV

have been reported in shelters and kennels in a number of states, suggesting the virus is spreading.  At the present

time, there are no vaccines licensedto prevent CIV.  Questions about the role of influenza virus or for that matter,

viruses other than CPI and CAV-2, bacteria other than Bordetella bronchiseptica, various mycoplasmas and other

factors causing kennel cough, which I refer to as "Canine Respiratory Disease Complex," exist and must be answered.

The geographic distribution of Lyme disease would suggest vaccination would only be of benefit in certain regions

of the US, thus widespread use of this product is neither necessary nor desired.  Although Wisconsin is an endemic

area for Lyme disease, we have used very few doses of Lyme vaccines in our VMTH and we have not seen significant

numbers of cases of Lyme disease.  However in certain areas of western and northwestern Wisconsin and eastern

Minnesota, many cases of confirmed Lyme disease are seen in both vaccinated and unvaccinated dogs.  Tick control

for prevention and antibiotics for treatment must be used in high risk areas, even in vaccinated dogs that develop

signs of disease (eg arthritis ) ...  Immunity to Lyme vaccines have been shown experimentally to last up to one year.

 Giardia is a new vaccine that may be of value in certain circumstances, but there have not been adequate field

studies to demonstrate the need nor the benefit of this vaccine.  To date no one has demonstrated a benefit for

coronavirus vaccine.  In the vaccination guidelines from the American Animal Hospital Association, neither Giardia

nor Coronavirus vaccines are recommended unless they can be proven to be beneficial for a specific animal. 

There are also new vaccines for snakebites ( Crotalus sp.) and for periodontal disease ( Porphyrius sp. )

and a therapeutic vaccine for treatment of canine melanomas.  New vaccines will continue to be produced and

licensed.  They are likely to be optional vaccines, thus their use will be determined from a risk/benefit analysis.

At present most canine core vaccines are given more often than needed, but a few non-core vaccines probably not

often enough to be of benefit.  Also, many vaccines/vaccinations are given that are not needed or that cannot be

shown to provide a benefit for the specific animal.  Vaccines are medical products that should only be given if needed

and only as often as is necessary to provide protection from diseases that are a risk to the health of the animal. 

If a vaccine that is not necessary causes an adverse reaction that would be considered an unacceptable medical

procedure, thus use only those vaccines that are needed and use them only as often as needed. 

Vaccination programs are changing and they will continue to change.  The vaccination program must be tailored to

the individual animal.  Vaccines are medical products that should not be used as practice management tools.   

My general philosophy is to vaccinate more animals in the population, but vaccinate with only those vaccines that the

animal needs and only as often as required to maintain protective immunity.  With some products, vaccination may

only need to occur once or twice in a life time, whereas with other products vaccination may need to be every 6 to 9

months, or at the very least annually ...

Be wise and immunize, but immunize wisely !

Why Vaccination Programs are Changing

Why, when you know from personal experience that life-long immunity exists for many human vaccines, do you have

great difficulty believing a canine vaccine can provide life-long immunity?  Perhaps I and my colleagues that teach

immunology to veterinary medical students have failed to explain the basics of vaccine induced "immunologic

memory."  Immunologic memory is as the term implies the immune system’s ability to remember the vaccine

antigens that it has seen at an earlier time in life, allowing the immune system to respond in a manner that will

protect you or your dog from specific infections and/or diseases.

Immunologic memory is responsible for the duration of immunity that develops after recovery from natural

infection/disease and after vaccination with modified live virus ( MLV ) recombinant ( r ) or killed virus ( KV )

vaccines.  Similarly bacterial infections and vaccines or bacterins ( killed bacterial vaccines ) provide immunologic

memory.  However, in general, immunologic memory to viruses is longer than to bacteria and memory to killed viral

vaccines and to bacterial vaccines ( or bacterins ) is not as long lived as it is to MLV vaccines.  The duration of

immunity or length of immunologic memory varies among the agents causing the diseases and the vaccines used to

prevent the disease.  For example, immunologic memory in humans for measles virus is life-long.  How do we know

that it is lifelong ?  No one has published any controlled studies, but we know after recovering from measles infection

and/or vaccination with a MLV vaccine, immunity is life-long because people rarely get measles even though they

rarely receive another dose of vaccine.  In contrast to the MLV vaccine, the killed measles vaccines that were used

for a short period of time about 25 years ago failed to give life-long immunity.  Many individuals receiving killed

vaccines were either inadvertently infected or had to be revaccinated with a MLV when they were 15 to 20 years of

age to provide life long immunity.  How many people do you know that were vaccinated with the modified live

measles virus product, in use for approximately 40 years, or that had measles as a child, then developed measles

later in their life?  I’m sure your answer must be very few or none ! 

A very similar story to measles can be told for canine distemper virus ( CDV ) in the dog.  CDV is in the same virus

family as measles virus and it shares many similarities with MV.  As you may know, MV vaccines were available until

recently for dogs to prevent disease ( not infection ) caused by CDV.  Those of you over the age of 50, may remember

canine distemper when it was a devastating disease killing many animals with more than 50% of infected puppies

often dying from the disease.  If you are old enough, were observant enough and had an opportunity to follow dogs

that recovered from natural infection with CDV you know that dogs recovering from CDV, like their human counterpart

recovering from measles, rarely, if ever, developed acute distemper during the rest of life, even when not

revaccinated.  Like measles immunity in humans, immunity from canine distemper infection confers immunologic

memory resulting in life- long immunity ...  How do I and my older, wiser and now retired colleagues and canine

infectious disease experts, Dr. Max Appel, Dr. L.E. ( Skip ) Carmichael, and Dr. Larry Swango know that distemper

immunity is life long ?  We know because we had the opportunity to follow dogs that recovered from infection with

CDV or puppies that were vaccinated once or twice with MLV CDV and lived for 7 or more years and never developed

disease even though they were exposed to CDV via natural outbreaks or experimental challenge with CDV.  We also

know the vaccinated or recovered dogs had life long immunity because we and others performed antibody tests for

years on the dogs after they recovered from infection or after puppy vaccination.  These dogs all had antibody

showing that immunologic memory was present and they were protected from disease.  Most of the dogs had enough

antibody that provides sterile immunity ( protection from infection ) much like the measles titers found years later

in many vaccinated or naturally infected people.  However even if the dogs didn’t have sterile immunity but have

antibody, they have immunologic memory.  An antibody titer, no matter how low, in a previously vaccinated dog over

4 months of age shows the animal has immunologic memory since memory effector B cells must be present to produce

that antibody.  Some dogs without antibody are protected from disease because they have T cell memory, that will

provide cell mediated immunity ( CMI ).  CMI will not protect from reinfection, but it will prevent disease. 

When an animal is antibody negative it may have T cell immunologic memory, but I consider a CDV antibody negative

dog not to be protected, therefore, I recommend revaccination !  Some researchers, including myself, have had the

opportunity to follow the duration of immunity for dogs living in natural or experimental environments that are free of

CDV and CPV-2.  Why is it important that observations are made on dogs and cats that are not exposed to the virus ?  Because in those environments it is possible to demonstrate that immunologic memory is independent of natural or

overt stimulation with the wild type virus or the vaccine virus.  However, in a normal environment where infection

occurs, "natural vaccination" or exposure and infection with the specific agent can and does occur at least for certain

agents and in certain animals, but the animals getting infected do not get sick.  Ironically when animals have "sterile

immunity" their immune system is rarely boosted by natural exposure since infection does not occur.  If infection

does not occur, there is no stimulation of the specific memory T or B cells, thus the antibody titer does not increase. 

A severe outbreak of CPV-2 occurred in a large commercial breeding kennel, where more than 90% of puppies got sick

and 50% of puppies from 4 weeks to 24 weeks of age died.  However,  none of more than 50 dams with sick and dying

puppies had a significant increase in antibody titer, none had virus in their feces and none showed clinical signs of

CPV-2 disease, all excellent indicators the dams had sterile immunity ( did not get infected ) !

Is immunologic memory and duration of immunity to all human viruses life-long ?  The answer is NO ! 

Natural infection with many human viruses and the vaccines for those viruses  provide life-long immunity

( e.g. measles, mumps, rubella ), whereas other viruses and/or the vaccines for them provide short duration of

immunity (e.g. human cold viruses, influenza virus ) and for additional viruses there is no immunity from infection

or experimental vaccines ( e.g. HIV ).  Similarly, immunity to human or canine bacterial vaccines may be long lived

or may be short, depending on the vaccine.

The three most important viral infections of dogs all provide life-long immunity, they are CDV, CPV-2, and CAV-1

( immunity provided by CAV-2 vaccine ) ...  If a puppy is immunized with the three MLV vaccines used to prevent

these diseases, there is every reason to believe the vaccinated animal will have up to life-long immunity ! 

The vaccines that prevent the diseases caused by these 3 viruses plus rabies vaccine are the "Canine Core Vaccines"

or those vaccines that every puppy should receive.  My own dogs, those of my children and grandchildren are

vaccinated with MLV CDV, CPV-2, CPI,  and CAV-2 vaccines once as puppies after the age of 12 weeks.  An antibody

titer is performed two or more weeks later and if found positive our dogs are never again vaccinated.  I have used

this vaccination program with modifications ( CAV-2 replaced CAV-1 vaccines in 1970’s and CPV-2 vaccines were first

used in 1980 ) since 1974 !  I have never had one of our dogs develop CDV, CAV-1 or CPV-2 even though they have

had exposure to many dogs, wildlife and to virulent CPV-2 virus.  You may say that I have been lucky, but it is not
luck that protects my dogs, it is immunologic memory !

An important factor contributing to life long immunity in addition to the memory T and B cells and the "memory

effector B cells" ( long lived plasma cells ) of the specific ( adaptive ) immune system is the innate immune resistance

associated with age.  It is well known in all species that the young animal is more susceptible to infection and disease

than a mature animal.  In the case of human infections that period of increased susceptibility is often the first few

years of life, and especially the first year.  In the puppy and the kitten it is often the first 3 to 6 months of life, but

it can be up to 1 year of age that the animal is more susceptible to disease.  For example, dogs less than a year of

age are much more likely to develop severe parvoviral disease than susceptible ( immunologically naïve ) dogs over

one year of age even though at both ages the animals are very susceptible to infection with CPV-2.  Similarly a

susceptible cat less than one year of age and especially cats less than 3 months of age are at much greater risk of

becoming persistently infected with feline leukemia virus than a susceptible cat that is greater than one year of age

at the time of infection.  Thus innate (natural) as well as specific immune factors contribute to age-related resistance

and these factors are highly complex and not completely understood.  However, age related resistance plays a critical

role in life-long or long term immunity.  This does not imply that older dogs and cats cannot get infected and develop

disease, it is that they are much less likely to get disease if infected with certain pathogens as compared to the

younger animal that gets infected. 

I and my colleague, Dr. Fred Scott, first proposed a three year revaccination program for dogs and cats approximately

30 years ago, when we published an article in Veterinary Clinics of North America 8(4) 755-768, 1978.  Today, a not

more often than three year revaccination program has been recommended in the AAHA Canine Vaccination Guidelines

and the American Association of Feline Practitioners Vaccine Guidelines for Cats.  The proposed change for

revaccination with "Core Vaccines" from annual to a not more often than triennial revaccination has been very

controversial for many reasons, however, the reasons have little or nothing to do with "immunologic memory" or

duration of immunity.  No one has nor can anyone in the future, show that there is any immunologic benefit from

annual revaccination with MLV CDV, CAV or CPV-2.  In fact, it may even be difficult to show an immunologic benefit

for revaccination at three to five year intervals since most animals have long term immunity for CDV, CAV-1 and

CPV-2.  Some among you are probably convinced that there is life long immunity to certain vaccines used in dogs and

cats, but few of you after many years of performing annual revaccination are willing to take the risk, however small

it may be, to adopt my puppy vaccination program.  However, you should feel confident that adopting, a three year

revaccination program for CDV, CAV and CPV-2, will not increase the risk for diseases caused by these 3 viruses,

just as a once every three year revaccination, rather than annual revaccination, with the killed rabies vaccines does

not increase the animal’s risk for rabies.

You and your veterinarian will need to determine what vaccines and vaccination program is best for your pet and their

patient respectively.  The program selected may only include core vaccines that are given once in the lifetime of the

dog or the program may include all vaccines with revaccination on an annual or more often basis, or it may be

a vaccination program in between these two extremes depending on what your pet’s needs are and what, in the

medical judgment of your veterinarian, is best for their patients.  Furthermore, it is likely the decision will depend

on the life style of your pet, its medical history, health status, age, pregnancy status and other important factors.

Suggested Reading :

AAHA Canine Vaccination Guidelines – 2006

www.aahanet.org/PublicDocuments/VaccineGuidelines06Revised.pdf

AAFP Feline Vaccination Guidelines – 2006

http://www.aafponline.org/resources/guidelines/2006_Vaccination_Guidelines_JAVMA_ PDF_Plus.pdf

WSAVA Vaccination Guidelines – 2007

http://www.blackwell-synergy.com/doi/abs/10.1111/j.1748-5827.2007.00462.x

Schultz, R.D. Duration of immunity to canine and feline vaccines : a review ...  - 2006

Schultz, R.D. Current and future canine and feline vaccination programs. 

Vet Med 93, 233-254. 1998.

Schultz, R.D., Conklin S. The immune system and vaccines. 

Comp Cont Educ Pract Vet 20, 5-18.  - 1998 ...

Schultz, R.D.  Considerations in designing effective and safe vaccination programs for dogs. 

In: Carmichael, L.E. (editor), Recent Advances in Canine Infectious Diseases. 

International Veterinary Information Service, http://www.ivis.org

Frequently Asked Questios ( FAQ ) :

From the Vaccination Guidelines Group ( VGG ) of the World Small Animal Veterinary Association ( WSAVA ),

published in the Journal of Small Animal Practice 48 (9) , 528–541, 2007.

1

Is there a risk of over-vaccinating a pet ( e.g. injecting it too often, or using vaccines that are not required for the specific pet ) ?

Yes - Vaccines should not be given needlessly, as they may cause adverse reactions.

Vaccines are medical products that should be tailored to the needs of the individual animal.

2

May I mix different types of vaccines in the syringe?

No - One should never mix different vaccine preparations in the syringe unless specified by the data sheet.

3

May I co-inject different vaccines ( not part of a single commercial product ) into the same animal ?

Yes - but different vaccines should be injected into separate sites that are drained by different lymph nodes.

4

May I use smaller vaccine doses in small breeds to reduce the risk of adverse reactions ?

No - The volume (e.g. 1.0 ml) as recommended by the manufacturer generally represents the minimum

immunizing dose, therefore the total amount must be given.