Veterinarians sometimes ask me, “Can I give a half dose of a vaccine to small breed dogs?” But more often they say, “An owner wants me to give their small dog a half dose of a vaccine because they are worried about adverse effects. Should I do that?”

The short answer is there is no evidence that decreasing the vaccine dose will have any impact on adverse events, and no data to know what impact it will have on the desired immune response.

Here’s a bit on the longer answer. The key issues are:

• We don’t know if lower doses of vaccine work in dogs and cats.
• We don’t know if lower doses of vaccine reduce the risk of adverse events.

It’s possible that lower doses would work and be could carry a lower risk of adverse effects. It’s also possible that lower doses would not provide good protection against the disease for which the vaccine is being given, and would still carry a similar risk of adverse effects, meaning we’d be getting little benefit from the vaccine while maintaining risk. That’s not good.

Why are vaccines dosed per dog (or cat, or person), rather than based on size like other medications?

Vaccines are not like drugs that are dosed to try to reach a certain concentration in blood or tissue, which requires higher doses for larger individuals. Vaccines predominantly work locally in the body, at or near the vaccination site. It doesn’t matter if the individual is big or small, the vaccine still does its work in a small location. We get similar immune responses to the same dose in large or small individuals.

That said, there are some data on the impacts of different vaccine doses on vaccine response and adverse effects. There’s no consistent association, as the vaccine type, target disease and age of the vaccinated individuals all play important roles in response. While we often focus on animal size, age may be a bigger factor and one that’s not usually on the radar.

• In humans, the approach to vaccination sometimes varies based on age (so indirectly on size, but age is the focus). Vaccines designed for older individuals sometimes have higher antigen loads because the immune response weakens with age. For example, there’s a high dose flu vaccine for seniors, and an older shingles vaccine had 14 times the antigen load of the version used to vaccinate kids against chickenpox (both caused by the same virus). Those products were designed based on concerns that older individuals need more antigen to properly stimulate the immune response, not that we want to use lower doses in smaller individuals.
• In contrast, the Tdap (tetanus, diphtheria, pertussis) vaccine for young kids actually has more antigen compared to that for older kids and adults. Lower doses for older individuals are meant to reduce adverse reactions, but that’s based on age, not size.

Do lower vaccine doses work, and do they reduce adverse events in pets?

We don’t know, since it hasn’t been studied properly in dogs and cats. While we have to remember that dogs are not people, and that vaccines used in different species also differ, as do the approaches to vaccination (especially frequency of vaccination), we can gain some insight on this from human studies.

A small study of pre-term infants looked at the effects of some physicians using half-dose vaccines (Bernbaum et al. 1989). They showed that 96% of pre-term infants that got the full dose of diphtheria/tetanus/pertussis vaccine responded adequately, while only 45% that got a half dose responded adequately (even after 3 doses), and an extra full dose was required to generate an adequate response.

In contrast, there are a few studies looking at half doses of COVID-19 vaccines, but it’s important to bear in mind that these use different vaccine technology to what is currently used for animal vaccines. COVID-19 vaccine developers also may have erred on the side of extra antigen because of the need to get these vaccines to market ASAP during the pandemic. So it’s a bit of an apples-to-oranges comparison, but shows that it’s fair to look at what happens with lower doses. A few studies showed that lower doses are likely protective, but that there’s no clear evidence that there’s an impact on adverse events.

• One study (Valim et al. 2022) was a non-inferiority trial that aimed to see if a half dose of the ChAdOx1 COVID-19 vaccine was no worse than a full dose. (That’s the Astra Zenica vaccine that was rolled out initially but was largely phased out when RNA vaccines became available). They showed that the half dose was non-inferior (23.7 vs 25.7 cases of illness per 1000 people/year). The frequency of adverse events was also similar. So, this the lower dose of the vaccine was similarly effective as the full dose, with no difference in safety profile.
• Another study of the same vaccine (Galvao-Lima 2023) showed that two half doses were similarly protective as two full doses, in terms of prevention of moderate to severe COVID-19 disease. There was no reporting of adverse event rates, so we can’t really interpret much from this.
• A small study of the Pfizer COVID-19 vaccine (Batmunkh et al 2024) looked at antibody response and adverse effects to full or half doses in people who had already received an initial COVID-19 vaccine series (any type). They found that the half dose was non-inferior to the full dose in terms of the antibody response for most individuals, but it was not as good for those who had received the Gam-COVID-Vac (Sputnik vaccine) for the initial series. Safety data were pretty limited, and the analysis was weak, so I don’t think we can conclude anything about that particular aspect, but it’s more evidence that COVID-19 vaccines can be effective at lower doses.

The first take-home message is that lower doses of vaccines probably work quite well – at least for some vaccines in some populations (I wouldn’t be as confident about the effectiveness of lower doses in seniors or other individuals who might have a harder time responding to a vaccine).

• That makes sense, since manufacturers don’t aim to find the absolute lowest antigen load that will work in the average person. They want their vaccines to work for the vast majority of the population, across the gamut of immune responses. Many or most individuals would response to less antigen than what is in the vaccine, but some probably do require the full dose – we just can’t easily tell who needs what.

The second take-home message is that there’s not really any clear evidence that reducing the vaccines dose reduces adverse effects. It seems logical that it would, if adverse effects are linked to the magnitude of the immune response and if the magnitude of the immune response varies with dose. However, since vaccines largely work locally, there’s nothing indicating that size is a component of this, and age probably plays more of a role. In dogs, breed does too. While we know that certain things may increase the risk of an adverse event after vaccination, we don’t have much supporting data that dropping the vaccine dose by a half helps to decrease the risk.

Do we have any data regarding using lower vaccine doses in dogs and cats?

No. Some people point to a 2015 study by a controversial veterinarian (who has been cited for practicing without a license on multiple occasions, markets unvalidated tests and pushes various narratives that are well away from the mainstream) (Dodds, AHVMA 2015).  It looked at half dose distemper/parvo vaccination vs full dose in 13 adult dogs (with only 8 dogs tested at some points because they somehow “inadvertently discarded” many samples). The study is small, very weak, doesn’t mention any ethics approval, and it’s unclear if validated tests were used – so it really can’t tell us anything. Even if the data are valid, we have to remember that this involves older (3-9 year-old dogs) that had been previously vaccinated with vaccines that are really effective, so they have a very well primed immune response.  But even then the data aren’t convincing. They showed that some (not all) dogs had increased titres after half dose vaccination, but there’s no control group that received the full dose vaccination, no group that was unvaccinated, and no statistical analysis. They also didn’t look at adverse events. So, I don’t think we can say anything at all from this study, which leaves us with no real data about half vs full dose vaccine administration in dogs and cats.

Is there liability for veterinarians if they don’t administer a full vaccine dose?

Veterinarians are expected to act professionally and reasonably, which includes communicating with owners and discussing options about their animals’ care. However, veterinarians are not expected (and in fact are expected NOT) to simply do what owners request if it’s medically (or ethically) inappropriate. Regulations and legalities vary by region but in general, owners cannot consent to malpractice, so client consent is not an automatic “get out of jail free card” if a veterinarian does something wrong. Even if the client thinks it’s a good idea and specifically requests it, the veterinarian is the professional and must use their expertise to assess the situation and what is or is not appropriate for the patient, so there is a professional risk to the veterinarian as well. Is giving half-dose vaccines malpractice? I’m not a lawyer so I can’t say, but I can think of some important considerations:

• If a veterinarian gives a half dose of rabies vaccine and issues a rabies certificate or indicates the pet is “properly” vaccinated against rabies (according to the manufacturer’s instructions, which is part of the regulation in Ontario), I’d consider that fraud and malpractice.
• If a veterinarian gives a half dose of another vaccine and implies that it will work equally well as a full dose, that’s a big concern. It’s using a vaccine off-label without consent.
• If a veterinarian gives a half dose of another vaccine and tells the owner it may or may not work, that’s probably defensible, but it would be a grey area. If the dog later had a problem (e.g. got sick) and the owner complained, the question might be how well the veterinarian really discussed the issues and risks. Owners can’t give informed consent if they are not adequately informed. If the veterinarian made the recommendation (versus giving into the owner’s request), then I suspect the liability increases, since a veterinarian’s recommendation/suggestion would reasonably be taken by an owner as an indication that the approach is safe and effective.

What should veterinarians do when pet owners are concerned about adverse events after vaccination?

The first and most important step is communication – that’s where things often fall apart. We have to acknowledge that vaccine adverse events are real and can happen to any dog, but are more common in certain breeds (especially small breeds). However, these event are still uncommon, and serious adverse events are quite rare.

A very large study of dogs reported an adverse event rate of 0.19% (19.4 adverse events per 10,000 visits) within 3 days of vaccination (Moore et al. 2023). The serious adverse event rate was even lower, around 2.9/10,000. Rare isn’t zero so we still have to be aware of the risk, but we also have to keep it in perspective. If reducing the dose of vaccines reduced adverse events by let’s say 15% (just to pick a number), that would drop the serious adverse event rate to 2.5/10,000. That’s an absolute risk reduction of 0.4/10,000 visits. We’d need to give 25,000 dogs a half-dose vaccine to prevent one severe reaction, but it’s possible we’d be trading that for an equal or greater increase in risk from the disease we’re trying to prevent with the vaccine. We just don’t know.

If an owner is concerned but their pet has never had a vaccine reaction before, talking about the overall low risk and the measures that can be taken to detect and address vaccine reactions is a good start.

If the individual animal has had a vaccine reaction in the past, then we should do a risk assessment and tailor the vaccination program for that pet to help decrease the risk. That could entail no longer vaccinating against all diseases, no longer vaccinating against some diseases (e.g. stop distemper/parvo vaccination but continue for rabies and lepto), changing vaccination regimens (e.g. splitting vaccines over multiple visits), vaccinating after pre-medication, or continuing to vaccinate as per usual, but being ready to respond if there’s a problem.

Is the issue of vaccine reactions in small breed dog actually related to size or breed?

That’s an important question, since I think many people often think “the dog is smaller, so the dog should get a lower dose,” like we do for drugs. It’s a logical inference, but if we look at the Moore study, breed played a big role. The table below shows the differences in adverse event rates between breeds. Small breeds dominated the highest risk breeds, but whether that’s because of their size or genetic factors is far from certain. Since some larger dogs cracked the top 10 list as well, we have to consider that genetics play an important role, and that size may just be along for the ride.

We can say that small breeds are higher risk for adverse events, but that’s not necessarily because they are small, it might be because of their genetics. Disentangling those is tough. If anything, we probably should explore safety and efficacy of lower doses in higher risk breeds (not just “small dogs”), and look at the impact of age on vaccine response too.

Key points:

• Half dose vaccines likely work in some, if not most, animals, but we don’t have any data to provide any confidence in that hypothesis.
• We have nothing to suggest that lower vaccine doses reduce adverse events.
• Higher adverse event rates in small breed dogs might be in part due to their size, but genetic factors likely play a big role.

Ultimately, I think using half doses of vaccines is bad medicine, since we are creating risk (potential for a poorer response, albeit low, which creates higher risk of disease) in the absence of any reasonable evidence that there’s a corresponding benefit (of any sort, let alone one that results in a net benefit).