# LHC Publications

April 30, 2021 by Suzanne Judd, LHC Director

Herd immunity occurs when the level of immunity in a population is high enough to prevent the spread of an infectious disease. In other words, when the number of people who are immune to a disease reaches a certain level, the disease no longer has as many chances to spread. This is best achieved through vaccination. In the case of SARS-CoV2, most researchers still agree that at least 70% of the population needs some level of immunity to slow the spread of COVID. One researcher has even started to use “herd immunity” interchangeably with “normality”. On the other hand, several other scientist are questioning if herd immunity is even an achievable goal. Another way to think of herd immunity is to consider it to be the level of immunity others in the community around a person have that will provide a buffer to keep the virus from rapidly spreading.

## Calculating Herd Immunity

Calculating the percentage that provides herd immunity is tricky and requires that researchers have first calculated R0, or the rate of the disease’s transmission in a population with no natural immunity to a particular virus. In order to calculate R0, we need data on the rate of case transmission before anyone in the population was infected or vaccinated. The estimates for R0 range from 0.8 – 10, which means herd immunity ranges from 50-90%. There are certain populations where diseases spread more rapidly, such as congregate living situations or those who spend long periods in places social settings where food.

Table 1: Range of Possible Herd Immunity Levels for SARS-CoV2 Based on Observed R0

## Measuring Herd Immunity

Cases are decreasing in Alabama, which is certainly wonderful news! However, this does not mean that we have achieved herd immunity; we still need Alabamians to keep getting vaccinated. Research data suggests that the human body may not have lasting immunity from SARS-CoV2 infection. This means that some people who had immunity from a previous infection (symptomatic or asymptomatic) may not have the same level of immunity today. In order to maintain the level of immunity we currently have in Alabama – which is working to reduce the spread of SARS-CoV2 – we need to continue vaccinating about 100,000 people each week.

## Estimating Current Level of Immunity in Alabama

Immunity to SARS-CoV2 could be achieved in three main ways:

1. Immunity without infection – People have antibodies even without a positive test due to asymptomatic infection or lack of test availability early in the pandemic. These people may have had symptoms or may not have had any symptoms. We have no idea how long the immunity lasts in people who did not have symptoms. Early data suggests that immunity in this group may fade as soon as 90 days after antibodies were detected in their blood.
2. Infection – People become sick, test positive, and then recover, having gained antibodies. This is the least desirable way to achieve immunity because some of the people who become sick will never recover. This approach leads to death, hospitalization and long term illness for some of the population.
3. Vaccination – This is the most desirable approach. This leads to the greatest benefit in terms of keeping people from getting sick and potentially dying. The complication rate from the vaccine is not zero but it is extremely low.

Figure 1 Model Estimating When Alabama Might Experience Herd Immunity

### Model Assumptions

1. At least 70% of the people in the State of Alabama will receive a COVID vaccine in 2021 (number will be easier to achieve if FDA clears the vaccine for 12-15 year-olds)
2. Vaccination is relatively similar across age, race, sex, and geographical location of residence
3. For every 1 positive test, there are 4 individuals who were not tested but have immunity (from undiagnosed, likely asymptomatic, cases)
4. Case rate will continue at current level
5. Vaccine provides some level of protection against variants
6. Immunity begins to fade after 6 months and by 12 months those with previous SARS-CoV2 infection are susceptible to reinfection
7. 30% of those vaccinated already had some level of immunity (this is the primary reason the green curve decreases when vaccinations ramp up)

### Which of these assumptions could cause the most problem if they are not met?

1. Vaccination rate: If at least 70% of Alabamans, do not receive the vaccine, community immunity levels will decline. By November of 2021, the total level of immunity in Alabama is estimated to be very similar to where it was in November of 2020 meaning the holiday season might look similar to last year.
2. Vaccination is similar by group: In Alabama, the group that is least likely to pursue the vaccine currently are those aged 16-30. This is also the group with risk behaviors that make them most likely to be in situations where SARS-CoV2 could rapidly spread. SARS-CoV2 does has a lower rate of mortality for young people compared with older individuals but there is now way to know if future variants will behave in the same way. Previous coronavirus pandemics (SARS and MERS) demonstrated a higher mortality rate in young people (1 in 50 aged 25-34 died). Failing to vaccinate 70% of the population could place unvaccinated groups such as children in a very dangerous place if the virus mutates and becomes more lethal.
3. Number of people with immunity but no positive test (asymptomatic individuals): Right now the bulk of the immunity in Alabama is from people who have immunity but have not had a positive COVID test and have not received a vaccine. If there are actually fewer of those individuals in the population, we are only at about 50% of the totally population with some level of immunity which is no where near high enough to stop the spread.

## Conclusions and Next Steps

Overall we are in a good place in Alabama at the moment because:

However, if we do not continue to vaccinate at least 100,000 people per week through November 2021, we could be tossed right back into an outbreak. This would force us to make hard decisions about whether or not we can remain open, allow children to go to school in person, and hold major events, which is a place no one wants to return to.