COVID-19 Booster Shots, Case Rates, and Holiday Gatherings

People working in a conference room while wearing masks during COVID-19

Written by WorldClinic

October 21, 2021

Booster Shots

Boosters have been authorized across the board in the United States. You still need to wait until next week until these boosters are widely available, as the CDC Advisory Committee on Immunization Practices still needs to issue the actual instructions for vaccine providers, but that should happen by the middle of next week although, in practice, many individual providers will just follow the FDA authorization and begin providing boosters right away.

Exactly what happened on Wednesday was that the US FDA adopted the recommendation of the Vaccine Advisory Committee and announced the authorization of boosters for both Moderna and Johnson & Johnson vaccines in the United States, meaning that all US authorized vaccines now have boosters. As the FDA said last week, they wanted to be as consistent as possible, so those who have been fully vaccinated with any one of the three vaccines authorized in the US can get a booster under certain conditions. For all three, the following groups can get a booster:

  • 65 years of age and older
  • 18 through 64 years of age at high risk of severe COVID-19
  • 18 through 64 years of age with frequent institutional or occupational exposure to SARS-CoV-2

The first two categories are straightforward, but the third is more ambiguous. The intent is to boost people like food service workers, teachers, and healthcare workers, but others with high direct work-related contact with many people in public settings appear to qualify, including those who must frequently travel and meet with outside entities. Additional guidance is expected next week from the CDC after the Advisory Committee on Immunization Practices meets on Tuesday.

Note that the Pfizer booster is the same dose as the original vaccination and the Moderna is a half dose.

For the two mRNA vaccines, the booster can be given no earlier than 6 months after completion of the primary series, so if you got your second dose of Pfizer or Moderna on April 1st, you can get your booster anytime on or after November 1st. For recipients of Johnson & Johnson, however, anyone who received the single dose of the Johnson & Johnson vaccine can receive a booster after only 2 months. The authorization is not for only certain groups but for every adult who received Johnson & Johnson.

Here is the additional authorization that was discussed last week, but was not expecting quite this fast: You can get your booster with any of the authorized vaccines, meaning you do not have to stick with the same vaccine for the booster dose. This is not a trivial authorization, because data from both the US National Institutes of Health and multiple international studies show that a mixing strategy is, in some cases, far superior to sticking with the same vaccine, in terms of average antibody levels achieved. This is most marked for those who received a Johnson & Johnson vaccine initially. Johnson & Johnson followed by Moderna resulted in a 76-fold increase in antibodies; Johnson & Johnson followed by Pfizer, a 35-fold increase, and followed by a second Johnson & Johnson only a 4-fold increase. If you got Johnson & Johnson, you should consider getting an mRNA booster. The data is not quite as clear in the other direction, but an mRNA followed by Johnson & Johnson is at a minimum non-inferior and the jury is still out on whether it may have a higher response rate..

So that’s who can get a booster, but do you have to or even need to. First, you do NOT have to get a booster to still be considered fully vaccinated. The case against getting a booster is that for all three vaccines, even with Delta, you are well protected against serious disease or death. But not perfectly protected. As part of the booster approval process, the FDA reviewed hospitalization rates for “fully vaccinated” people and those rates do modestly increase being farther out from the original vaccination. If this is due to waning immunity or the recent surge in cases due to Delta is the subject of some debate, but there is no debate that infection rate does increase being farther out from initial vaccine completion. Additionally, the data from Israel is very compelling. Delta had caused a marked new wave in both cases and hospitalizations, but once they started a broad booster program, cases declined over about 6 weeks to less than 10% of the peak. Boosters worked and without significant observed safety issues.


To be sure, there are concerns about auto-immune issues, including transient myocarditis, with the mRNA vaccines and rare clotting issues with Johnson & Johnson, so the big question that the FDA and CDC have wrestled with is whether these known risks are less than the risks of bad outcomes from breakthrough disease. The assessment of the outside panels is that is the case: Risks favor getting vaccinated. The FDA is mandating a specific warning regarding heart inflammation in young males who receive Moderna, and women of child-bearing age who receive Johnson & Johnson, but risks have otherwise been relatively limited.

Assuming the CDC makes no major changes, which is not expected, we encourage anyone over 65 or with an underlying medical condition to get a booster, and from the data we have seen, it appears that Moderna may be a marginally better choice if you have the opportunity to choose. For younger people, unless the CDC guidance is much stricter than expected, if you are in a position that does require frequent contact with the public, including frequent travel, then we would also recommend getting a booster with the caveat to avoid Johnson & Johnson in women of childbearing age and Moderna in 18-24-year-old males.

Case Rates

Moving to case rates around the world, the most important point is that just about all of the indicators are going in the right direction. Cases in the United States are down about 40% in the last month. By the CDC’s newest criteria on transmission rates, seven states are now at a rate below what the CDC has labeled as high transmission although 22 states are in the intermediate range by the criteria we have used through most of the epidemic in the United States. The WHO notes that every region of the world is seeing case rate decreases except Europe, but in reality, most of Europe is on a similar downward trajectory, with the exclusion of a couple of eastern European countries including Russia, where COVID case rates are high enough that there is even more increasing evidence of social disruption.

As discussed last week, one of our greatest concerns has been why the Covid rate in the United Kingdom has remained stubbornly high. The rate continues to increase to near the mid-summer peak. Compare this to the rates in Spain, France, and Germany that have all come down to less than 10 cases per 100,000 per day and less than 5 in the case of Spain. There is reason to think that the United Kingdom is peaking but this bears watching.

In Asia, rates are near negligible in Japan and Hong Kong, while Singapore, South Korea, and Australia, which had been seeing spikes, appear to have turned the corner with rates coming down

Holiday Gatherings

In considering how to safely have a holiday gathering, the biggest question is how to hold an event at reasonable risk in order to minimize the risk of any infections and not to risk becoming a super-spreader event. The best approach is to consider a “multi-layered” risk reduction strategy, sometimes referred to as a “Swiss cheese strategy.” The assumption is that no mitigation approach is 100% effective, but if multiple mitigations are applied, the “hole” in one layer will be handled by another layer, and depending on the strength of each layer and number of layers (mitigation strategies) applied, it may be reasonable to omit one or two layers of mitigation without unreasonably increased risk. You also have to keep in mind that if the goal of the event host is to eliminate the chance that anyone could get infected, then the only good option is to not hold the event. If there is a willingness to accept a small risk of infection with an exceedingly small risk of an infection that leads to a bad outcome, in exchange for the typically intangible benefits of holiday gatherings, then a gathering can be held at reasonable risk.

We cannot provide a bright line across the board guidance statement on what you should do, but we can lay out the seven most obvious layers of prevention to be considered:

First, the Background level of transmission: This is the single most important determinant of risk, but it is completely uncontrollable by the planner. It is reasonable (but not a sure thing) to believe that, in the absence of a new variant, the overall case rate in many areas of the US will be below 50 cases per 100k per week (this is “moderate” transmission risk or better) by early December and at least below 100 cases per 100k per week (“substantial” but not “high”) in most major cities.  At moderate or below, CDC says that masking is not needed by vaccinated people in public areas (i.e., areas where the vaccination status of others in the area cannot be known), so this would be considered a reasonably low-risk environment for a gathering. “Substantial” risk is not insignificant but can be overcome by the application of other mitigation tools.

Vaccination: For a voluntary gathering, as in a holiday party, it is not unreasonable to require vaccination for all those eligible for vaccination. Transmission in a fully vaccinated environment is significantly less likely than in a mixed environment with both vaccinated and unvaccinated. If children who are not yet eligible for vaccination are included at the event, the risk is raised only minimally, especially if the unvaccinated children are required to mask for the function (it is likely, however, that vaccination for children ages 5-12 will be available over the course of November, so the number of fully unvaccinated children should be small, although it is not likely that any children under 12 would be fully vaccinated by early December).

Symptom screening: One of the most powerful tools in reducing transmission in a group setting is ensuring a positive individual confirmation of lack of symptoms prior to the event. While there is an asymptomatic infectious period, with the Delta variant this period is much shorter than earlier in the epidemic. Especially for a voluntary event, it is very beneficial to remind people that even mild symptoms can represent COVID (or other upper respiratory infections) and that it’s not worth taking a chance.

Masking: The CDC guidance is still that masks should be worn in any public indoor space if the local transmission risk is “substantial” or higher (greater than 50 cases per 100K per week – 7.14 cases per 100K per day averaged over 7 days). Note that this is for a “public” indoor event, implying that the vaccination status of those attending is not known. For a group known to be fully vaccinated, it is reasonable to be more lenient on masking (although there is no formal guidance on this). For a known fully vaccinated group, bumping up one level in the CDC guidance is reasonable. This would mean that if the local level is below 100 cases per 100k per week, no masks would be required if everyone is vaccinated. Many metropolitan areas are at that level now. If the level is higher than 100 cases per 100k per week (“high” transmission), then masks should be required for all attendees. If the group consists of mixed vaccinated and unvaccinated, then everyone should wear a mask in any area with a case rate greater than 50 per 100k per week. Over the next 6 weeks, it is not clear how much of the US will make it below this level.

Indoors versus outdoors: Outdoors events (with air movement) have essentially infinite dilution of airborne viral particles. For events that can be held outdoors, this dilution power overcomes many other risks. While it is possible to contract virus outdoors if standing close to someone spewing COVID virus (before distance and dilution can affect the “plume”), this is much less common as demonstrated by the lack of significant COVID transmission evident at football games with tens of thousands of people shoulder to shoulder and front to back, loudly cheering and singing. Indoors versus outdoors is perhaps the largest controllable factor by an event host.

Air management: Very closely related to the outdoor versus indoors factor is the role of indoor ventilation and filtration. There are three factors at play here: fresh air makeup (fresh outdoor air exchanged for potentially contaminated indoor air), air filtration (the presence of air filters in line with air movement through or into the room), and airflow rates. With knowledge of these parameters combined with the volume of the room and the expected number of people, an HVAC engineer can determine whether the ventilation adequately and expeditiously reduces the density of any airborne particles. This is somewhat controllable by the host through the choice of room size versus attendance since modification of existing HVAC systems is not an overnight action. Maximization of airflow and filtration in conjunction with a facility operator should be confirmed prior to any indoor event.

Testing: The CDC prefers avoidance of the possibility of human-to-human interaction for avoiding infection, but CDC also endorses a testing strategy if contact is unavoidable. With Delta, a PCR test can identify an asymptomatic infection roughly a day before the person becomes infectious, while antigen testing is only useful to tell you that someone if infectious or not at the time of testing. The addition of a testing program for all attendees is another mechanism for reducing the risk of introducing virus to an event. “Day of” home antigen testing, either by self-report or enforced (e.g., picture of the test result) adds minimal cost/hassle. PCR testing takes much longer (typically at least overnight and at a much higher cost. For planning purposes, PCR starts at $100 (often higher), while antigen testing starts at $7-10.

While this is not an exhaustive list of the tools that a party sponsor can use to reduce the risk of infection to an acceptable level, a strategy that chooses how many of these tools and how they are applied can allow for relatively safe events.


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