COLUMN: Ask Dr. Jim Halverson: Understanding herd immunity

web 4 17 Halverson photoBy Dr. Jim Halverson, Special to the Ojai Valley News

It has been six weeks since the March 20 Stay-at-Home order. We continue to have only seven confirmed cases in the Ojai Valley due to your great efforts to follow Public Health guidelines. Thank you!

Predicting how long we will have to live with the SARS-CoV-2 virus and its threat to vulnerable people is difficult. Ultimately, it comes down to one factor — the development of herd immunity.

Herd immunity

This is an easy concept to understand — one of few with this virus.

Take 100 cows. Introduce a cow-specific virus into the herd. Let's call it Cow Cough Disease (CCD). CCD is particularly dangerous to the older cows in the herd. Its survival depends on being spread from one cow to another.

If CCD is a highly contagious virus (like the measles), one cow can give it to 15 other cows. If those cows survive, they become immune. If it is mildly contagious (like the common cold), one cow can give it to only three cows.

Again, if they survive, they become immune. The development of herd immunity depends almost entirely on the contagiousness of the virus. For the herd to be protected from a mildly contagious virus, only about 50% of the cows will need to be immune. For a highly contagious virus, up to 95% will need to be immune.

COVID-19 is a moderately contagious disease. It is predicted that 60% to 70% of our American population will need to be immune to it so that the virus can no longer survive by finding easily available non-immune humans. Ultimately, the same percentage of people in the world will need to be immune to essentially eliminate it as a health threat worldwide.

Immunity is obtained though the development of antibodies in a human against the specific virus. Those antibodies can only be present if they are given to the patient (temporary passive immunity being tried for critically ill patients), are produced by a person to help survive the disease, or produced by people who are vaccinated.


Current "best-guess" estimates are that approximately 5% of the U.S. population is now immune to SARS-CoV-2. The immunity may be somewhat higher in New York, which has had significant disease burden and lower in our valley, which has had very little disease. To reach the goal of 60% to 70% immunity,  an effective vaccine is needed.

Vaccine development

A vaccine that can prime the body to build an army of antibodies and immune cells trained to recognize and destroy the coronavirus will ultimately prevent COVID-19. The World Health Organization says approximately 70 vaccines are currently being developed against COVID-19. Unfortunately, it will be a year or more before the most safe and most effective of those vaccines are ready for general use. And that's with an already shortened timeline, thanks to new technology that enables scientists to make vaccines from the digitized genetic sequence of SARS-CoV-2, instead of having to grow large amounts of the virus itself. Doctors have already begun testing vaccines in the first healthy volunteers. Ultimately, it will likely be 12 to 24 months before this virus is no longer a serious health and financial threat to our world.

We must adapt to the virus being in our communities. Stay-at-home restrictions will eventually ease, but stay-safe-and-stay-well recommendations will not. Use this time (we have plenty of it) to do the following:


1) Follow a healthy diet. Do not use the these restrictions as an excuse to gain weight (the COVID-"19 pounds"). If you do drink, do so in moderation. Poor diets and excessive alcohol are very bad for our physical and emotional health.

2) Exercise. I clarified with our Public Health Director Robert Levin, M.D., that stay-at-home orders do not exclude outdoor exercise activities. People of all ages are allowed to be out for walking, biking, hiking or jogging. If you can maintain at least 6 feet of separation from others, masks are not required.
3) Get adequate sleep. Try for at least seven hours. What else do we have to do at night?
4) Stay in contact with family, friends, your church community and others through telephone or other forms of communication. Visit with others following recommended social-distancing guidelines.
5) Continue to follow health providers' recommendations. Offices are open and safe to go to. Telehealth visits can be arranged if you feel safer at home.
6) Respect others, especially those at the greatest risk of COVID-19. Twenty percent of our valley (6,000 residents) are 65 or older. This generation worked tirelessly to provide a better life for the rest of us. Let's take better care of them.

— Dr. Jim Halverson is a longtime Ojai Valley physician, who is providing regular updates on COVID-19 for the Ojai Valley News.

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