What is herd immunity?

Joshua Krisch
Live Science
Originally published July 2020

Here is an excerpt:

Herd immunity doesn't always work

The ingredients for achieving herd immunity naturally are well understood. "You want a disease that is guaranteed to produce robust immunity with largely asymptomatic spread, and have a low R0," Altmann told Live Science. But even if the R0 is relatively high and most patients are symptomatic, herd immunity is still possible with an effective vaccine, and a vaccine program that immunizes the population en masse. "Think of our big, public-health vaccination success-stories: Smallpox and polio, both entirely due to massive, sustained vaccine programs with simple, highly effective vaccines," he said.

Robust immunity is necessary to ensure that those who become immune stay that way long enough for the pathogen to die out. Asymptomatic spread helps, because it means that fewer people are likely to die while the population waits for herd immunity to take hold — and increases the likelihood that there will be enough survivors to affect herd immunity in the first place. A low R0, of course, lowers the bar for how many individuals need to be immune before we see the infection rate flatten and decline.

Nonetheless, some diseases that are seemingly strong candidates for herd immunity never quite achieve it. Despite widespread infection and vaccination, chicken pox, for instance, has never been entirely eradicated from the population. That's because the virus that causes chicken pox remains latent in the nerve roots of those who are infected by it, even after they recover and acquire immunity to the disease. As once-infected individuals grow older their immune systems weaken and the virus can reactivate, causing shingles, which can, in turn, cause chicken pox.

"You might have eradicated chicken pox in a small island community, but then somebody's granny gets an attack of shingles and, over a matter of weeks, every kid on the island gets chicken pox," Hunter said. "You've achieved herd immunity, and [it appears] the virus has died out, but it's actually waiting to come out." Similar phenomena have been observed with tuberculosis, according to the WHO.

Vaccine-induced herd immunity can also fail when a vaccine results in only short-lived immunity within a population. Pertussis and mumps recently reappeared long after vaccine programs were assumed to have eradicated these diseases, and studies suggest that, while vaccine noncompliance played a role, the outbreaks were in part due to the vaccines losing effectiveness over time. "In the past few years we've had both pertussis and mumps outbreaks, and those have primarily resulted from waning immunity over time," Poland said.

The info is here.

How Trump failed the biggest test of his life

Ed Pilkington & Tom McCarty
The Guardian
Originally posted 29 Mar 20

Here is an excerpt:

Those missing four to six weeks are likely to go down in the definitive history as a cautionary tale of the potentially devastating consequences of failed political leadership. Today, 86,012 cases have been confirmed across the US, pushing the nation to the top of the world’s coronavirus league table – above even China.

More than a quarter of those cases are in New York City, now a global center of the coronavirus pandemic, with New Orleans also raising alarm. Nationally, 1,301 people have died.

Most worryingly, the curve of cases continues to rise precipitously, with no sign of the plateau that has spared South Korea.

“The US response will be studied for generations as a textbook example of a disastrous, failed effort,” Ron Klain, who spearheaded the fight against Ebola in 2014, told a Georgetown university panel recently. “What’s happened in Washington has been a fiasco of incredible proportions.”

Jeremy Konyndyk, who led the US government’s response to international disasters at USAid from 2013 to 2017, frames the past six weeks in strikingly similar terms. He told the Guardian: “We are witnessing in the United States one of the greatest failures of basic governance and basic leadership in modern times.”

In Konyndyk’s analysis, the White House had all the information it needed by the end of January to act decisively. Instead, Trump repeatedly played down the severity of the threat, blaming China for what he called the “Chinese virus” and insisting falsely that his partial travel bans on China and Europe were all it would take to contain the crisis.

The info is here.

Changes in risk perception and protective behavior during the first week of the COVID-19 pandemic in the United States

T. Wise, T. Zbozinek, & others

PsyArXiv
Originally posted 19 March 20

Abstract

By mid-March 2020, the COVID-19 pandemic spread to over 100 countries and all 50 states in the US. Government efforts to minimize the spread of disease emphasized behavioral interventions, including raising awareness of the disease and encouraging protective behaviors such as social distancing and hand washing, and seeking medical attention if experiencing symptoms. However, it is unclear to what extent individuals are aware of the risks associated with the disease, how they are altering their behavior, factors which could influence the spread of the virus to vulnerable populations. We characterized risk perception and engagement in preventative measures in 1591 United States based individuals over the first week of the pandemic (March 11th-16th 2020) and examined the extent to which protective behaviors are predicted by individuals’ perception of risk. Over 5 days, subjects demonstrated growing awareness of the risk posed by the virus, and largely reported engaging in protective behaviors with increasing frequency. However, they underestimated their personal risk of infection relative to the average person in the country. We found that engagement in social distancing and hand washing was most strongly predicted by the perceived likelihood of personally being infected, rather than likelihood of transmission or severity of potential transmitted infections. However, substantial variability emerged among individuals, and using data-driven methods we found a subgroup of subjects who are largely disengaged, unaware, and not practicing protective behaviors. Our results have implications for our understanding of how risk perception and protective behaviors can facilitate early interventions during large-scale pandemics.

From the Discussion:

One explanation for our results is the optimism bias.  This bias is associated with the belief that we are less likely to acquire a disease than others, and has been shown across a variety of diseases including lung  cancer. Indeed,  those  who  show  the  optimism  bias  are  less  likely  to  be  vaccinated  against disease. Recent evidence suggests that this may also be the case for COVID-19 and could result in a failure to engage in behaviors that contribute to the spread this highly contagious disease.  Our results extend  on  these  findings  by  showing  that behavior  changes  over  the  first  week  of  the  COVID-19 pandemic such that as individuals perceive an increase in personal risk they increasingly engage in risk-prevention  behaviors.   Notably,  we  observed  rapid  increases  in  risk  perception  over  a  5-day  period, indicating that public health messages spread through government and the media can be effective in raising awareness of the risk.

The research is here.