By Dr Eddy Bajrovic*
Heard of herd immunity? Let me explain, because it’s important whether you’re travelling overseas or staying at home.
Think of all Australians as being part of one (very big) herd.
When any disease – let’s say, measles – comes along, those herd members who’ve had it before or have been vaccinated against it are protected through their individual immunity.
Should measles visit them again in the future, their immune system recognises the virus and they generate measles antibodies to kill it before it can make them sick.
High levels of childhood vaccination in Australia over decades mean that most members of our ‘herd’ are immune to the outbreaks of infectious childhood diseases that used to kill kids and adults by the thousands last century.
As a result of childhood immunisation, our national immunity to vaccine-preventable diseases is generally strong, so that occasional outbreaks don’t spread far.
(Unfortunately, herd immunity doesn’t protect us against seasonal influenza. Flu strains are constantly changing, so the annual vaccine formula needs to be updated each year in both the northern and southern hemispheres.)
Vulnerable people rely on safety in numbers
However, not all members of our herd are protected – or can be.
Some people refuse to be vaccinated, relying on herd immunity to protect them and/or their children. Others are either too young to get the vaccine (under 6 months), have an existing chronic illness, and/or their advanced years render any past vaccines less effective.
Past vaccination also may not protect someone whose immune system is weakened by a condition such as leukaemia or HIV-AIDS, or from the effects of chemotherapy. Nor can they receive a live vaccine, which contains a weak form of the virus and could potentially be harmful.
When diseases break out, those in these vulnerable categories can’t develop sufficient antibodies to protect them from illness, which is also likely to be more severe for them.
Nor do vaccines work for 100% of recipients – even a healthy person may not develop immunity to a particular vaccine. It’s estimated that around 5-15% of recipients may not initially respond to the first 3 doses of Hepatitis B vaccine they receive, which is why those people who rely on its protection in their profession are routinely advised to have their antibody levels or ‘titres’ checked.
It’s because of these vulnerable members of the broad Australian ‘herd’ that national vaccination levels need to be maintained as high as possible.
Herd immunity important for travel, too
So, what’s herd immunity got to do with overseas travel? More than you might think.
The level of herd immunity can vary considerably from one destination to the next. Not so much in other developed countries with high vaccinations levels, but certainly in developing countries where rates of immunisation may be much lower and large-scale outbreaks more likely to occur.
To understand the risk low herd immunity poses to travellers, it helps to understand that the way a disease spreads determines how many unprotected people it’s likely to infect.
As we know, many diseases are spread by an infected person coughing, sneezing – even breathing – into the air, including in places like aircraft cabins, airport terminals, buses, trains, and busy hotel lobbies.
But, not all airborne diseases are alike.
Some, such as measles, chickenpox and tuberculosis are transmitted by breathing in very fine aerosolised particles that can remain afloat for several hours – well after the infected person has left the same space. This trio is among the most infectious of all diseases: each new case can result in 10 others. (In contrast, people with the deadly Ebola virus on average infect around 2 others.)
So, vaccination levels and herd immunity need to be high to protect against measles and chickenpox (there is no effective TB vaccine for adults).
Other airborne diseases, such as the cold and flu viruses and the meningitis bacteria, are spread via larger droplets that remain airborne for a much shorter period of time. For that reason, they require close, direct face-to-face contact or touching an infected hard surface.
Because each new case of these diseases is likely to infect only a few people at most, not quite as many need to be vaccinated to prevent large-scale outbreaks.
Foreign travel brings risk of foreign diseases
For a variety of reasons, these vaccine-preventable childhood diseases have made a comeback in recent years. Even developed countries like Australia, which were officially declared free of measles years ago, have seen a growing number of local outbreaks recently – many the result of a traveller returning home infected.
Last year, Australia recorded 339 cases of measles – virtually all of them linked to overseas travel. In one well-documented outbreak, a young adult who contracted measles abroad sparked 170 cases here in 2012.
It’s the reason Travelvax Australia echoes the recommendation of the Australia’s medical experts and government health authorities that all travellers visiting any country – developed or developing – ensure their childhood vaccinations are up to date well before you leave home (6 weeks is ideal). These include measles, diphtheria, whooping cough, mumps, tetanus and polio, plus the seasonal flu vaccine.
For travellers there is the added potential for infection with food- and water-borne diseases such as typhoid, cholera, and viral hepatitis that we rarely see in Australia, banished by efficient sewage treatment an effluent disposal, and high levels of sanitation and hygiene.
Conversely, the absence of those high living standards leads to a deadly cycle of disease perpetuated in many developing countries. When outbreaks inevitably occur, these diseases spread far and fast to potentially contaminate the food or drinking water unwary travellers consume and the everyday objects they might touch.
Fortunately, many of the potential disease risks travellers can encounter are preventable through vaccination, plus simple prevention measures such as choosing the safer food and beverage options and effective hand hygiene.
Vaccination: Both smart and responsible
Firefighters know that creating firebreaks prevents a more devastating bushfire.
In medicine, we know high immunisation rates create protective firebreaks around the most vulnerable – those who will not or cannot be vaccinated.
If you’re travelling, immunisation not only protects you, but also fellow passengers, other guests at your resort or hotel, and your family, friends, and community contacts back home.
These people aren’t made ill, hospitalised – even killed – and health authorities need not use up precious resources tracking down and treating everyone you, and those you’ve unwittingly infected, have come into contact with.
Home or away, getting vaccinated is both the smart and the responsible thing to do for the good of our herd.
* Dr Bajrovic is the Medical Director of Travelvax Australia.
Routine, recommended and required travel immunisations – along with expert tailored advice and information – are available during a pre-travel medical consultation at Travelvax Australia’s clinics nationwide. For advice or to make an appointment, please call 1300 360 164 (toll-free from landlines) during business hours.