Herd Immunity

I recently pulled my old (copyright 1998) epidemiology textbook off the shelf and checked several internet sites to find out what percent of the population needs to be immunized to achieve herd immunity.  Ideally, 100% of the population would get vaccinated.  When you achieve that level of immunity the virus cannot find a host and stops spreading.  Eventually, the virus may die out or mutate into another, hopefully less lethal form of the virus.  That is what happened to the H1N1 variant that caused the Spanish Flu outbreaks of 1918 and 1919, which is the ancestor of the seasonal flu bug.  We’ve achieved nearly 100% immunization for polio and measles, which are now almost totally eradicated in the U.S. and much of the world.  There have been outbreaks of measles among children in the few places in the United States where parents are opposed to the MMR vaccine, but they are rare and totally preventable.[1]

When it comes to communicable disease, the best offense is a strong defense, which is how vaccines work; they rev up our immune system to fight disease naturally.  Rather than treat the virus after you have been infected, the vaccine helps our body’s own natural defenses identify the unhealthy, invading virus and then destroys it before it makes us sick.  All of the vaccines currently being administered reduce the risk of getting the virus, reduce the severity of illness if you do get sick, and reduce mortality.  Thus it prevents us from getting sick from the virus now and because the immune system recognizes the invading virus, it will protect us when exposed to the virus in the future.  And by protecting us it protects the people around us as we are less likely to carry and spread the virus to others.  Herd immunity is obtained when large numbers of the population are immunized, making it difficult for the virus to find an unvaccinated host.[2]

To get back to normal, i.e. where we don’t have to wear masks, don’t have to social distance, and can participate in group activities including going to the movies, restaurants, and sporting events, between 90% and 95% of the U.S. population will need to be vaccinated against COVID19.  Numerically, that would still leave between 16.5 and 33 million Americans unprotected and capable of catching and spreading the virus and of course dying from it.  Health officials are hoping that 80% of the population gets vaccinated by late summer and possibly 90% or more by the end of 2021 as recalcitrant and hard to reach individuals come forward.  Not including children under the age of 15, who are less susceptible of dying from the disease, a 90% adult immunization rate translates into 245 million Americans needing to be vaccinated.

Over 30 million Americans have tested positive for the virus and most medical experts agree that the real number is probably twice as large because the majority of infected people are asymptomatic.  The good news is that if you were infected you have built up some immunity.  The bad news is that this natural immunity does not appear to last as long as the one created by taking the vaccine.[3]  Additionally, since most people don’t know that they’ve had the virus, (between 65% and 85% of those who contract the virus are asymptomatic) they wouldn’t know if they have immunity.  Therefore, to the extent possible everyone – adults, teens, and children – should get vaccinated. 

Because the virus mutates into different strains so easily health officials also believe that booster shots will become the norm, much like the seasonal flu.  And like the seasonal flu and the viruses that cause the common cold, Covid19 isn’t going away, it will just lack the number of unvaccinated hosts to spread easily.  The millions who do not get vaccinated will remain at risk for catching the virus and will be more susceptible of dying from the disease.

Now that the process for administering the vaccine is moving full steam ahead, the health community is focusing on the reasons a large number of Americans are resisting getting the vaccine.  In January, a frequently cited survey indicated that about a third of Americans had either decided to not get the vaccine or were still hesitant about getting it.[4]  The major findings from that survey found:

  • Nearly two in five adults were hesitant about getting a COVID-19 vaccine.
  • Male, older, white, married, and higher socioeconomic status individuals more likely to vaccinate.
  • Republicans and Fox News viewers were less likely to vaccinate.
  • Being currently immunized against influenza predicted COVID-19 vaccination intent.
  • A better understanding of COVID-19 vaccination hesitancy is needed.

Resistance to taking the vaccine also appears to be higher among African-Americans who are concerned that the government is not being honest about the side effects.  This is concerning because people of color, including Black Americans, Hispanics, and Native Americans have higher death rates from COVID19 than white Americans.[5]  Fortunately, the Black community appears to be responding to education and appeals from Black leaders.  Unequal access appears to be the main reason so many people of color are slow to receive the vaccine, which the Biden administration is trying to address.

Hesitancy among conservatives comes from distrust in government, to lesser extent due to hoaxes and conspiracy theories directed at conservatives, and unfortunately, due to misleading information coming from right wing media and Republican legislators.  Fox News hosts Sean Hannity and Tucker Carlson have both pushed the line that taking the vaccine is a personal decision and people have the right not to take it.  For example, in a uniquely hypocritical statement, Hannity told his viewers he planned to take the vaccine but also said people shouldn’t feel shamed or pressured to get vaccinated.[6]  However, that line of thinking defeats the purpose of public health.  Using that same rationale, we could also make the case that individuals shouldn’t have to pay taxes that provide for clean drinking water and sewerage treatment, the two public health initiatives that have eradicated cholera and dysentery.  

Hannity and Carlson have also raised questions about the safety of the vaccine.  Carlson told his viewers, “it turns out there are things we don’t know about the effects of this vaccine, and all vaccines, by the way.  It’s always a tradeoff.  But in this specific case, Germany, France, Italy and Spain, these are not third world countries, have just suspended the distribution of AstraZeneca’s vaccine.”[7] 

All medications have side effects; in rare cases even aspirin can cause death.  And yet millions of doses of aspirin are taken daily because the benefits greatly outweigh the risks.  The same is true for all of the COVID19 vaccines, where a few serious reactions have occurred and in a few rare cases even death, but where it will save millions and millions of lives.  Using Carlson’s reasoning, people should never go outdoors because in a few cases someone will get stung by a bee, go into anaphylactic shock and die.[8]  This is why the manufacturers of the vaccines conducted phase 3 trials with tens of thousands of volunteers – to assess the effectiveness of the vaccine as well as potential side effects in samples approximating large populations.  And in the case of the AstraZeneca vaccine, researchers quickly isolated why it caused blood clots in a few individuals and identified effective treatment for that adverse reaction.[9]

Finally, there are still a large number of Republicans in Congress who have not taken the vaccine or refuse to do so; about 25% of Republicans as I write this.  In addition to being hypocritical – they are praising that effective vaccines were developed while Trump was in office but then refuse to take it – they are also being poor role models for those individuals who are hesitant about taking the vaccine: white, male Republican voters. 

For well over a century, life expectancy has increased nearly every year in the United States.  Living healthier and longer is primarily due to innovations in medical technology, pharmaceutics

(especially antibiotics), and public health initiatives.  Cholera, dysentery, yellow fever, malaria, polio, venereal disease, tuberculosis, influenza, and typhus killed hundreds of thousands of people each year in the U.S. prior to these medical advances.  With over half a million deaths in the U.S. due to COVID19, now is not the time to turn our backs on effective medicine and public health.   

Despite these fears and concerns, the good news is that resistance to getting vaccinated against COVID19 is decreasing and an increasing number of Americans are enthusiastic about getting vaccinated.  Eventually, we will be able to be among others without wearing a mask and social distancing.  It also means that people will be able to return to their jobs and our economy and nation can once again prosper.  So, if you know someone who is hesitant about getting the vaccine, remind them that the health of a nation is dependent on the health of its people.


[1] 207,000 people died of measles worldwide in 2019, up from 140,000 in 2018.  The majority of deaths were of  children under the age of five.  Measles vaccination programs were hampered by the COVID19 pandemic, thus measles deaths are expected to increase for 2020.

[2] Here is a simple and concise explanation of vaccine efficacy: Why you can’t compare Covid-19 vaccines – YouTube

[3] How Long Does Immunity Last After COVID-19? What We Know (healthline.com)

[4] Predictors of intention to vaccinate against COVID-19: Results of a nationwide survey – ScienceDirect

[5] Black Vaccine Hesitancy Rooted in Mistrust, Doubts (webmd.com)

[6] Hannity Tells His Huge Audience It’s Fine Not to Get the Vaccine (thedailybeast.com)

[7] CNN’s John Berman Slams Tucker Carlson for Vaccine Remarks (thewrap.com)

[8] More people dying from wasp and bee stings – UF Health Podcasts

[9] Scientists Say They Found Cause of Rare Blood Clotting Linked to AstraZeneca Vaccine – WSJ

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