The role of vaccines in disease control and their potential to cause harm are issues very much in the media at the moment.
Much of what I read is confusing (what are spike proteins?), scary (blood clots – eek!) or vastly oversimplified (vaccines will fix the world).
I think we could be accused of the same problems with communication when it comes to pets. Do we avoid talking about complexity and uncertainty because these are difficult? Oversimplification is the easy road, but have we created unrealistic expectations about immunity?
Preventive medicine is a tricky thing. It's basically trying to predict the future risks of an individual within a population and doing things to reduce the risk ... without causing (too much) harm. Much of it's based on epidemiological analysis (ie the who, why and where of disease). On a population level, we can say, 'well this seems to work' after a period of study. But we can never really know if our preventive measures have been effective in an individual – would they have succumbed without it?
What makes things trickier is the basic terminology. I use the following terms every day, and still, find them difficult to explain:
infection
transmission
symptoms
immunity
vaccines/vaccination
Below, I’m going to try to provide some basic information – while pet and human immune systems are very similar and yes, pets get versions of coronavirus, this is not a discussion about COVID or the jabs that have been developed against it. That’s way, way out of my league.
What is an infection?
At its most basic, an infection is an illness caused by invasion and growth of a microorganism. Let's use the term germ from here on, it's easier.
The world is choc-full of germs. You and I and our pets are choc-full of germs. But we get relatively few infections. Why is that?
Because an infection results from the interplay between three ingredients:
a disease causing germ
a host (animal or person)
an environment
Every interplay between these three things is different. Each ingredient has many factors.
1 + 2 + 3 = a very big number of possible outcomes!
Fortunately, very few of the interactions we all have result in serious disease.
Germ factors
I'm using this term to cover the range of disease-causing pathogens including:
bacteria
viruses
fungi
parasites
prions
Each one of these has a whole branch of science dedicated to them. Germs have bits on them (or produce things) that the immune system recognises as foreign (and in need of destruction). These are called antigens.
On a very basic level, we can divide germs as primary or opportunist/secondary.
Primary ones can cause disease in a 'healthy' host. Their ability to cause disease depends on things like:
virulence (kind of like strength)
number
host's defences
A really virulent strain may be able to cause disease in a healthy host when just a few germs get into the body. Virulence is also affected by factors like environment (eg temperature, humidity) and mutations. A weaker germ can still cause disease if large numbers manage to get into the body.
Opportunistic or secondary germs cause disease in a 'compromised' host. This compromise may be due to a depressed immune system or 'special' access to the body (eg a wound).
For germs to 'successfully' cause disease, they must make it through 4 stages of infection:
colonisation – germs need to colonise a host by getting to an appropriate spot in the (eg lungs, intestines) and by having a mechanism to attach to the host surface
invasion – after the germs have colonised the host, they need a way to invade into the cells and cause damage to the cell, which allows them to take over the cell and trigger symptoms that allow transmission
proliferation – the germs hijack the cells and use the cells' nutrients and protein making abilities to make more germs
transmission – for germs to survive they need to find new hosts, so they need ways to get from one host to another (eg by getting into secretions or into poo and triggering sneezing, coughing or diarrhoea)
How successful the germs are at getting through these stages depend on their ability to hide from and/or fight the host's immune system. As time goes on, germs can adapt (mutate) so they can hide better.
Host factors
Host factors are the individual variations that affect susceptibility to infection. Whether a germ causes disease in an individual (and how severe that disease is) is affected by:
the host's ability to recognise the germ
the ability of the host to resist the germ
the damage the host's immune system does to itself while trying to control infection
And these factors are in turn influenced by many things including:
overall health, age, sex, diet and lifestyle factors
genetic factors
environment factors (eg climate, weather, pollution)
Just like the germs have stages of infection, hosts also go through different stages. These are:
incubation – the time from exposure to a germ to development of symptoms. During this time, germs are replicating inside the host. The incubation period varies with different germs.
prodromal – the period of time after incubation and before the characteristic symptoms of that infection occur. Non-specific symptoms such as low-grade fever and fatigue may occur and the host can be transmitting the infection.
illness – the period of time where the host shows symptoms of the infection (or has clinical disease). The host will be infectious during this time
decline – (not as bad as it sounds!) this is when the immune system mounts a (successful) defence against the germs and the number of infectious particles declines (decreases). Symptoms gradually improve but this is the time when secondary infections may occur. The host may still be transmitting the primary germ.
convalescence – during this stage, symptoms resolve but depending on the severity of infection, the host may have long-lasting or permanent damage from the germ itself or from the immune system reaction
A quick note: while the terms symptoms and signs are used to mean the same thing, they're actually not. Symptoms are things you can describe but can't really see (eg aches, pains, fatigue). Signs are things you can see and usually measure (eg fever, swelling). Strictly speaking, we shouldn't talk about symptoms in veterinary medicine as our patients can't describe anything to us.
What is immunity?
Despite my scientific background, immunology feels like scary magic (or astrophysics). If I try to grasp it, it both disappears and gets bigger and bigger. Here goes anyway!
Our immune system is hugely complex. But think of it as having two branches – one is there ready to go and the other one 'learns' and adapts over time. It's the second branch that's key when we're talking about vaccines.
Innate immunity
This is the host's first line of defence. It's a non-specific response 'ready-to-go' against any germ.
The main parts of innate immunity:
physical barriers to prevent colonisation (eg skin, mucus)
white blood cells (particularly neutrophils and macrophages) which attack any foreign invader
inflammatory mediators, which are released to attract more immune cells to the affected area
a sequence of reactions (called the complement cascade), which is activated to clear the germs and the damaged/infected cells
activation of the adaptive immune system, where 'antigen presenting cells' show bits of the germ to cells such as helper T cells and cytoxic T cells so the host can remember and be prepared for next time
While this part of the immune system is ready to go (in a healthy host), it's still relatively slow. This is part of the reason we have symptoms before we get over an infection.
We can also experience symptoms due to the immune response itself. While the white blood cells are attacking the germs, they are damaging the surrounding tissue and we can experience pain from this. Those inflammatory mediators being released can also cause fever. So those symptoms experience during the prodromal stage of infection may be due to the germs and/or due to the immune response.
Adaptive immunity
This is the specialised part of the immune system. It recognises and clears previously encountered germs – it's the part that learns so that 'next time, we're ready'.
There are two arms to the adaptive immune response:
cell-mediated response – this is where 'memory' white blood cells called helper T cells, B cells and cytotoxic cells defend against germs by binding to cells infected by germs (killing the cells with the germs in them)
anti-body mediated response – this is where 'memory' B cells make antibodies that stick to the germs that have not yet invaded cells, by doing this they either neutralise the germs or make them better targets for cells involved in innate immunity
Adaptive immunity is much faster at fighting off the germs – it's activated even before the germs invade the cells. It's not that you can't get the same infection twice, you just get better and faster at responding to it.
It's possible that acquired immunity can work before we show any symptoms or transmit germs. But, it's also possible (depending on germ, host and environment factors) that we do experience symptoms and/or transmit germs for a period of time despite 'having immunity'.
It also possible that the triggered immune response can go wrong and go off-target. When this happens, we can see healthy issues being attacked and damaged by mistake.
What is a vaccine?
A vaccine is a biological substance that creates/builds adaptive immunity to a specific germ. It aims to by-pass the slower innate response and go straight to acquired immunity if we encounter the germ.
Vaccines may contain:
live germs (rare these days)
dead germs
'attenuated' or inactivated germs
antigens from the surface of the germs
While these germ bits can't cause disease, the host's immune system doesn't know this and mounts an immune response.
In adults, booster vaccinations are given to increase the level of antibodies in the system.
The effectiveness of and symptoms associated with the immune response vary from vaccine to vaccine and host to host. Replace 'germ' with 'vaccine' and that interplay of ingredients we spoke of at the beginning is still at work.
What should we expect from vaccination?
The take-home message is that the development of immunity (whether from getting the germ or getting a vaccine) isn't absolute.
Vaccination is not some kind of impenetrable shield. It's a biological process with lots of factors. When it comes to pets, those factors are multiplied as we use vaccines against several diseases at once.
And while yes, we want to minimise the risk to your pet from serious disease, it's also a population or community thing. By vaccinating the healthy, we give the germs fewer new hosts to invade (or invade for as short as possible) and reduce the risk of those who are more susceptible.
We’re all just trying to be good humans!
