What You Need to Know-Week of August 24th

The most important weekly updates for you to keep your community healthy

In this Update:

1. Information You Need: Duration x Viral Shedding Level x Contact Distance = Infection

2. Information You Need: What is Herd Immunity, would it work for COVID-19, and what is the cost?

3. Mask Up and Make a Difference: “Pro” tips for face coverings

1. Information You Need: Duration x Viral Shedding Level x Contact Distance = Infection

What do the terms duration and “distance” and “viral shedding level” mean?

  • Viral shedding level – means the level of virus that the person you are in contact with is shedding or putting out into the air
  • Contact distance means the intimacy of the contact you have – how far are you away from them.
  • Duration means the amount of time that you have been exposed to an infected person.

What is needed for an infection to occur?

  • The recipe for a viral infection requires several things: contact with an infected person, the amount of virus they were shedding at the time and the amount of time exposed. When someone becomes infected with a virus it means they were exposed to enough viral particles for a long enough amount of time to cause an infection.

How does this apply to COVID-19?

  • For the COVID-19 virus, those particles are mainly spread through respiratory droplets that are produced by breathing, talking, sneezing, and coughing. This means that if you are in close contact (within 6 feet) with someone who is currently infected with COVID-19 (whether they are showing symptoms or not), you are most likely breathing in their respiratory droplets. However, for you to become infected you must be in close contact with an infected person for long enough to inhale enough viral particles needed to make you sick. 
  • COVID-19 virus can spread over longer distances so even if you are six feet away from someone there is a chance you can still become infected. This transmission is called airborne transmission. This happens more when you are indoors, in a small room without good ventilation, cold conditions and when the person who is sick is breathing hard, coughing or singing. All of those things help the virus get into and stay in the air.

A general formula for infection is: Duration x Viral Shedding Level x Contact Distance

  • This amount of virus is called the minimum infectious dose and it varies by the type of virus and characteristics of the person. 
  • The amount of viral particles that will cause illness is different for everyone. For example, those who are more susceptible (e.g., older adults and immunocompromised individuals) to the virus can breathe in fewer particles than those less susceptible and still get infected. This means that they would need less time around someone who was infected to catch the virus compared to someone who is not part of a high-risk group for COVID-19.
  • With certain diseases, we know that an exposure can be directly related to disease severity. In other words, more exposure to the virus can mean a more serious case of the disease. Scientists are exploring whether or not this is true for COVID-19. 

To reduce the risk of developing COVID-19 you should:

  1. Reduce your exposures 
  1. Reduce the duration of time you are around people
  • Lessening the duration of time you spend around others will reduce the likelihood of inhaling enough viral particles to make you sick. 
  • For example, if you don’t like ordering groceries because you like picking your produce, order everything else for grocery store pick-up and then go inside for a shorter duration to get your produce.

2. Information You Need: What is Herd Immunity, would it work for COVID-19, and what is the cost?

You may have heard about herd immunity in the news recently. Herd immunity is an important public health concept. It refers to the reduced chance of the spread of many diseases, often vaccine preventable diseases like measles and polio. Reaching herd immunity without a vaccine means that a high percentage of the population has to be naturally infected. This could mean a high cost of people sick, hospitalized or dying from the infection.  

What is immunity?

  • If you are immune to a disease or virus it means that your body is able to fight off the germs that cause it so that you never become seriously ill and you can’t pass the virus on to other people. People gain immunity by either (a) being vaccinated or (b) by becoming infected and therefore developing antibodies that help the body resist invading germs that cause a particular disease. 

If someone contracts COVID-19, are they immune?

  • The most recent evidence suggests that antibodies to the virus last at least three months. There is less evidence related to if that prevents people from becoming ill, but we expect that there is at least some protection from these antibodies based on research with other viruses. There is also growing evidence that our bodies T-cells, another part of the immune system, can also provide protection. 

So what is herd immunity?

  • Herd immunity occurs when enough people in the population become immune to a specific disease, which limits the ability for the disease to spread to the small number of people who are not immune. Herd immunity can only occur if a large proportion of the population has been previously infected or vaccinated for a disease. 
  • However, this proportion changes depending on the disease or virus that is spreading. For example, roughly 95% of the population is needed to be vaccinated or infected with measles for herd immunity to occur, while only about 80% of the population needs to be immune to polio to reach herd immunity. This is related to how infectious a virus is. The more infectious it is (i.e. the easier it is to pass between people) the more people need to be immune to stop transmission.

Would herd immunity work for COVID-19 and what are its costs?

  • To reach herd immunity without a COVID-19 vaccine it is estimated that 60-70% of the population would need to be infected with the virus and develop antibodies that cause immunity. 
  • Some rough estimations are outlined below. Note that it is more complex than discussed and depends on the patterns of how people interact with each other and the risk factors in each population (age, health problems, healthcare access and treatment). But this gives a rough idea of the magnitude of potential deaths. 
  • In the United States it is estimated that 0.65% of those who are infected with COVID-19 will die. Therefore, to achieve herd immunity, in the United States 213,330,000 million people would need to be infected to reach the best estimate of 60-70% immune which would be estimated to cause 1,386,645 deaths.
  • By applying the United States estimates to Arizona, we assume that in order to achieve herd immunity, 4,876,930 million Arizonians would need to be infected, which is estimated to cause 31,700 deaths.
  • Globally it is estimated that 0.5-1% of those who are infected with COVID-19 will die. This number is differs in countries around the world due to their age of the population (older more at risk of death, level of health problems, sicker populations more at risk of death)  Therefore, to achieve herd immunity, more than 4.9 billion people worldwide would need to be infected, which could cause an estimated 36,750,000 deaths.
Total PopulationInfection Fatality Rate (Proportion of deaths among all infected individuals)Percentage of people that would need to be infected with COVID-19 to reach herd immunityNumber of people that would need to be infected with COVID-19 to reach herd immunityEstimated number of deaths that would occur to reach herd immunity
Arizona7.279 million (2019)0.65% (United States estimate)67%4,876,93031,700
United States328.2 million (2019)0.65% (crude)67%219,894,0001,429,311
Global7.59 billion (2018)0.5 – 1% (.75%) 67%5,085,300,00038,139,750
* These numbers and percentages are only estimates based on evolving COVID-19 data. The table is meant to demonstrate the difficulty and cost of reaching herd immunity without a vaccine. There are limitations to the estimations above because they are based on unadjusted infection fatality rates and do not take into consideration age or other influential factors that may impact the data.

3. Mask Up and Make a Difference: “Pro” tips for face coverings

Are you already in the habit of wearing your face covering? Take your mask-wearing to the next level with these “pro” tips:

Have at least seven washable face coverings for each day of the week, plus a box of disposable masks in case you forget or are unable to routinely wash your cloth face coverings. 

Find a mask with a removable filter layer because, often, the filter is not machine washable, but the rest of the mask is. This way, the filter can then be hand washed separately, prolonging the life of your mask. 

Ear loops can become uncomfortable, especially after an hour or more. If you know you’ll have to wear your face covering for hours at a time, invest in one with ties or an elastic band that goes around your whole head, rather than just your ears, for greater comfort. 

To create a tighter seal across your cheeks and nose, twist the ear loops once before putting them around your ears (it will look like a figure 8 from the side). This works extremely well for glasses-wearers, too, to reduce fogging! 

After you put your face covering on, take a moment to talk to yourself while looking in a mirror. For a good fit, there shouldn’t be any large gaps around the edges of the face covering when you’re speaking

If you can blow out a candle or lighter flame 6 inches in front of your face, your mask is too easy to breathe through! 

Do you wear glasses and struggle with fogging or slipping while wearing a face covering? 

Are you constantly touching your face to adjust your glasses? Here are special “pro” tips just for you:


  • Try improving the fit of your glasses by visiting an optometrist or purchasing anti-slip silicone ear grips. These grips are available to fit onto different parts of your glasses, for example nose pads and earpieces, and can be purchased as a kit to give you options to experiment and find the best fit for you. 
  • If a headband is an option for you, sew a button onto the headband and hook the ear loops from your mask into the button instead of your ears. 


  • Put your glasses on the outside of your mask. Not only will this help reduce fogging, but it will help keep your mask fitted snugly to your face – win/win! 
  • Try an anti-fogging glass cleaner. 
  • Adjust the ear loops or ties for a tighter fit: If your mask has ear loops, twist them once into a figure-8 before putting the loop around your ear. If your mask has ties, make an X: The string at the bottom of your mask should go high around the back of your head, and the string from the top of your mask should go low, tying closer to your neck. 

The next update will cover information on getting your flu shot. If you would like to learn more about this and other topics related to COVID-19 in Arizona, please complete next week’s AZCOVIDTXT survey that you will receive via text in about a week.

View Updates from Past Weeks:
Update from week of August 17th (English | Spanish)
Update from week of August 10th (English | Spanish)
Update from week of August 3rd (English | Spanish)
Update from week of July 27th (English | Spanish)
Update from week of July 20th (English | Spanish)
Update from week of July 13th (English | Spanish)
Update from week of July 6th (English | Spanish)
Update from week of June 29th (English | Spanish)
Update from week of June 22nd (English | Spanish)
Update from week of June 15th (English | Spanish)
Update from week of June 8th (English | Spanish)
Update from week of June 1st (English | Spanish)
Update from week of May 25th (English | Spanish)
Update from week of May 17th (English | Spanish)
Update from week of May 11th (English | Spanish)
Update from week of May 4th (English | Spanish)
Update from week of April 27th (English | Spanish)
Update from week of April 20th (English | Spanish)


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