The vaccinated herd

The herd is us.

We are the herd. Getty

The simple answer is, we are all the herd. At least with infectious diseases, we are all in it together.  Our health depends on the health of the many. If an infection spreads from person to person, it depends on who we have contact with and whether they carry an infection.

If many are immune to a disease, the herd is protective. Those who are not immune can be protected by their neighbors and friends who are immune.

If everyone is vulnerable to a disease, a disease may spread far and fast.

If not everyone is vulnerable, a disease can burn out. 

Protecting ourselves from disease, as a group

Firefighters know this. They create firebreaks. They dig ditches, carry out controlled burns. By removing vulnerable - combustible material - as a fire spreads, they stop a fire outbreak.

In medicine, we create firebreaks for disease outbreaks through vaccination. Those who are vaccinated - and those who've had the actual infection - are normally immune. If there are enough people who are immune - and in the right places - the disease spread can stop.

We specifically vaccinate certain age groups - such as adolescents for HPV, college students for meningitis, travelers for Hepatitis A. We may vaccinate heavily in certain geographic areas to create a ring of protection - such as for polio in Nigeria and Pakistan.

A large percentage of the population needs to be vaccinated in order to protect those who are not - or cannot - be vaccinated.


The herd protects those who need protection

In wildlife, we imagine the herd protecting the weak. In this case, the herd protects those with weak immune systems. 

Those who are most vulnerable to some diseases - those with weak immune systems - cannot be vaccinated. When you need the vaccine most, you can't have it.

Live vaccines (which have weak viruses) cannot be given to some people with weak immune systems, where even a weak virus can be harmful. Past vaccination may not even be protective if someone's immune system is very weak. Those who have immune disorders - especially leukemia, AIDS, or anyone receiving chemotherapy - need the protection of the herd, of the group, to stay safe. Such persons likely also go to hospitals and clinics frequently and may be more likely to be exposed to diseases they cannot individually protect themselves from. We need the group as a whole to be protective, by eliminating these diseases.

This applies not just to people who know they have weak immune systems. Not everyone can be immune to everything, even folks who are healthy. Some vaccines do not work in everyone. There may be healthy people who do not respond. About 5-15% may not initially respond to the first 3 Hepatitis B vaccines they receive. If they don't check with a blood test, they may not know they don't make antibodies.

 (Fortunately, the CDC reports only <5% of vaccinees do not respond to the vaccine 6 valid doses)

How protective the herd is depends on the disease

How many people need to be vaccinated depends on how the disease spreads, what sort of contact is needed, and how much it spreads per each contact.

If everyone were vaccinated, a disease wouldn't spread. If 95% are vaccinated, a disease probably wouldn't spread. If 90% are vaccinated, some diseases still wouldn't spread, but it depends on the disease. Some diseases wouldn't spread if 2 in 10 people you come in contact with are not vaccinated; others would still spread.

It just depends on how many people each infection spreads to. It depends on how the disease spreads to those who a sick person has contact with. Some diseases just infect one or two new people per each infectious case. Other diseases may lead to 10 or 15. How a disease spreads determines how many people will be infected. How many people would be infected without vaccination determines how many people need to be vaccinated to protect everyone.

Diseases spread in different ways

How many people need to be vaccinated depends on the disease - and how the disease spreads.

Some diseases spread in the air. Tiny, tiny particles of the disease can float in the air and spread to others unseen. Fortunately only a few diseases spread this way - but they tend to be very infectious. They can spread in some cases anonymously even before someone knows what they are sick with. These diseases include: Measles, Chickenpox, Tuberculosis. Measles can infect 10 people per case. Tuberculosis can lead to 10 new infections per case. Many people need to be vaccinated to protect against measles and chickenpox. (There is no effective TB vaccine for adults).

Some diseases spread by droplets, especially when we cough or sneeze. Many cold and flu viruses, as well as the bacteria that causes meningitis, spread this way. This generally requires being within 3-6 feet of an infected person, face-to-face. These do not usually spread as much as airborne infections. Each infection often leads to just 1 or 2 or 3 other infections. Many need to be vaccinated, but not as many as for chickenpox or measles.

Some diseases spread when what should go down the toilet ends up drinking water or in our food. Those of us who have good sanitation can worry less about many such fecal-oral diseases. However, for those at risk (which includes when people travel abroad), these diseases can spread far and fast - like Typhoid, Cholera, and Hepatitis A. There are vaccines for these but many around the world suffer from these diseases.

Some fecal-oral diseases though are so infectious they spread even when sanitation is good - like Norovirus, the cause of many stomach "flu"s in the US and around the world.  Other infections may spread by respiratory droplet or the fecal-oral route, but also spread by objects - a doorknob, a countertop. These infections include Enterovirus and Norovirus. If someone touches a doorknob when sick, someone else can unknowingly touch this doorknob and then their face, becoming infected. Unfortunately, there is not a vaccine for enterovirus or norovirus in the US.

The Herd can be protective if enough people are vaccinated. Diseases like measles or chickenpox need more people vaccinated to protect others. Diseases like meningitis and the flu need high rates of vaccination, but not as high as measles, to protect us.

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