Why so Few Zika Cases this Year? Learn Why, Learn About Herd Immunity, and Learn Why You Should Still be Ready
Many people remember the outbreaks global Zika outbreaks in 2015-2016 and recognize that the number of cases - and the hype - have died down.
The image below shows data from Central America, the Caribbean, and South America. The graph shows the number of Zika cases in each area over the 52 weeks of the year. Globally, the case numbers spiked in early 2016.
Zika cases in the Caribbean and South and Central America
Image from Pan American Health Organization (PAHO) & World Health Organization (WHO)
, using data from individual countries and territories
Last month, the
August 11 Zika newsletter
addressed the myth that Zika is no longer a threat (in Myth of the Week sidebar).
Although Zika may have decreased, one major reason is
The New Yorker recently published an article discussing some reasons for the decline of Zika, including herd immunity and climate change, which can bring Zika to more northern regions without herd immunity. (Herd immunity will be discussed below and climate change was also discussed in this newsletter's Zika Information section).
Read the New Yorker article
What is herd immunity?
Imagine this is a population, below:
None of these people have ever been exposed to Zika virus. Zika is a new, emerging disease that has not yet come to their part of the world.
The virus arrives for the first time and one person gets infected (shown in green). Remember, many people who have Zika will not have symptoms, so the person may not even know that he or she is infected.
Mosquitoes bite the infected person and become infected too. They spread Zika by biting other people (shown with yellow arrows). Those people become infected.
Zika can also be spread through sexual activity. The infected person also spreads Zika through sex (shown with the orange arrow). Now several people are sick.
It is especially easy to spread Zika in areas like South America because many people live in close quarters and do not have screens or air conditioning. In some of these countries, condoms - used to prevent sexual transmission of the virus or delay pregnancy to prevent Zika-related birth defects - may be
expensive or illegal
. Some women may also face
such as lack of adequate sex education and little control over their reproduction (including violence against women), or concerns about how to protect themselves while following their personal or religious beliefs.
These infected people also spread the virus through mosquito bites or sexual activity.
More and more people become infected.
These people can also spread the disease to others.
After a person has Zika, he or she becomes immune to the virus.
Immunity is protection from a disease. Immunity can occur if a person was infected by a disease and can not get the disease again. Immunity can also occur if a person was vaccinated for a disease (although this is not an option for Zika as there is currently no Zika vaccine).
When Zika virus is no longer in the body, the body's immune system can still "remember" it and can protect against future infection.
As time goes on, more people in the population become immune. This is especially true of a disease like Zika, since almost everybody will survive.
These people are shown in white. They cannot spread the disease and they cannot get the disease again.
The disease continues to spread and more people become sick - and then immune. Eventually, the population may look something like this, with most people immune:
Remember, white circles represent people who are immune. Blue circles represent people who are susceptible (not immune) to Zika virus.
When someone else becomes sick, Zika cannot spread very well in the population because most people are immune. There are fewer people to spread the disease and also fewer people to catch it. An immune person will not become infected when he or she is bitten by an infected mosquito or engages in sexual activity with an infected person.
Only a few people in the community can become infected. The high number of immune people can also protect many of the susceptible people (people who are not immune) from getting sick. A small number of people may still get sick, as shown below.
When you think of herd immunity, think of a herd of animals - such as sheep, or in this case, people. The "herd" can provide health protection if many people have immunity to a disease - in this case, Zika.
(Herd immunity is also why it is important to get recommended vaccines and vaccinate your children for other diseases, since this can protect other community members, including those who cannot be vaccinated for medical reasons.)
The images below show an overview of what happened in Latin America. This is one reason that Zika declined. Since no country is in a vacuum isolated from the rest of the world, this is also an important reason for the decrease of Zika in the United States this summer.
HOWEVER - this does
not mean that Zika is no longer a threat. Over time, there will be more young people born and they will be susceptible to Zika virus. Some of the currently immune people will get older and pass away (from unrelated causes). This leads to a population with less immunity and more opportunity for the disease to return. Most people in the United States are not immune, so Zika could also spread more easily in this population - although it is
currently less likely to come to the US because there of herd immunity worldwide.