April 05, 2021

Covid Cathy Bites the Dust

Now that just under half of older people have been fully vaccinated against Covid-19 and only about a quarter have not received any vaccinations at all,  the burning question is, what can vaccinated people do safely? 


The answer comes in two parts: what can vaccinated people do that does not jeopardize their own health and what can they do without risking harming others? The CDC has weighed in on this, focusing principally on the first issue, safety of the individual. Their guidance includes the recommendations that those who are fully immunized (who are at least two weeks out from their second shot) can visit other fully immunized people indoors without masking or social distancing and that they can travel without self-quarantining upon arriving at or returning from their destination.


To answer the second question, the public health concern, we need to know whether a vaccinated individual can be infected with Covid-19, remain asymptomatic, and transmit the disease to an unvaccinated person. Physicians have been concerned that while the antibody response to vaccination is highly effective in squelching the virus in the lungs, what’s not clear is whether it’s also effective in killing the SARS-Cov2 virus in the nasal passages. If so, vaccinated individuals could indeed be surreptitious sources of disease, like the notorious Mary Mallon, who was an asymptomatic carrier of the bacteria causing typhoid fever, salmonella typhi. Could asymptomatic Covid carriers act like “Typhoid Mary,” perhaps earning the nickname Covid Cathy? At last, we have very reassuring data addressing this issue.


The current issue of MMWR, the weekly journal published by the CDC, reports on the experience of just under 4000 people during the period mid-December and mid-March, 2479 of whom received two shots of either the Pfizer or Moderna vaccines and 989 controls who remained unvaccinated. They also report on 477 people who received one dose, but for simplicity, I will ignore these partially immunized individuals. The investigators leading this small study took one crucial step that has previously been largely lacking: they tested all the participants weekly using the gold standard polymerase chain reaction (PCR) test for the SARS-Cov2 virus—whether or not they had been vaccinated and whether or not they reported symptoms. What did they find?


The outcomes are reported based on “person-days” since the group who were vaccinated got their shots at varying times and therefore differed in the number of days they could have become infected. They found that among the fully immunized, the number of new positive tests/1000-person-days was 0.04 whereas among the unvaccinated, it was 1.38. The bottom line: once you are fully immunized, you are far less likely to test positive for Covid-19 than if you have received no vaccinations.


The study also found that only 23 percent of the people who did develop an infection got sick enough to see a physician, only two people were hospitalized, and no one died. 


Until this admittedly small but carefully conducted study appeared, it seemed to me that while vaccinated people could feel personally quite safe, they had to exercise caution in the interest of public health. It wasn’t really true that vaccinated people could socialize indoors with other vaccinated people—until the issue of Covid Cathy was resolved, they had to be worried about about any unvaccinated household contacts their friends might have, lest an asymptomatic carrier inadvertently transmit the virus to a friend, who while also asymptomatic, manages to give the virus to an unvaccinated household member. Now it increasingly looks as though this theoretical concern is not, in practice, of great consequence. 


Just because fully vaccinated individuals are reasonably safe today doesn’t mean they will necessarily remain safe tomorrow. Vaccine effectiveness is calculated based on how much less likely a vaccinated person is to get the disease than an unvaccinated one. But if the disease is running rampant in the surrounding community, that is, if it is quite common among the unvaccinated, then while the relative risk of the vaccinated will be unchanged, the absolute risk will go up. And if new variants appear against which the vaccines offer only limited protection, then the relative risk itself will be affected. 


So, keep an ear to the ground—monitor how common the virus is in the community where you live and pay attention to the type and pervasiveness of viral variants. If the situation is stable, enjoy your freedom.