Are we close to herd immunity?

a microbiology newsletter -may, 2021

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Are we getting closer to herd immunity?

As of May 2nd we now have a vaccination rate of 40% of the adult population in the US (4)

How far do we have to go to reach herd immunity?

First of all we must know that herd immunity is defined as a form of indirect protection from infectious disease that can occur with some diseases when a sufficient percentage of a population has become immune to an infection, either through vaccination or previous infections, thereby reducing the likelihood of infection for individuals who lack immunity.(1)

Although many different percentages have been declared to reach herd immunity, the WHO states the target percentage for COVID-19 is not currently known. This explanation is from the WHO website (2):

"The percentage of people who need to be immune in order to achieve herd immunity varies with each disease. For example, herd immunity against measles requires about 95% of a population to be vaccinated. The remaining 5% will be protected by the fact that measles will not spread among those who are vaccinated. For polio, the threshold is about 80%. The proportion of the population that must be vaccinated against COVID-19 to begin inducing herd immunity is not known. This is an important area of research and will likely vary according to the community, the vaccine, the populations prioritized for vaccination, and other factors."

However, in an effort to determine the herd immunity percentage, a recently CDC published study examined infection rates in ten communities in Qatar and concluded that an infection rate of 65 - 70% would be needed to reach herd immunity. Alternatively, herd immunity might be reached at a vaccination coverage of about 75% if vaccine efficacy is 95%, similar to that of the recently licensed SARS-CoV-2 vaccines, the researchers claimed.(3)

By Jay Hardy, CLS, SM(NRCM)

President, Hardy Diagnostics

References: 1, 2, 3, 4

Better Safe than Sore!

When it comes to Group A Strep, better detection methods are needed now more than ever!

A sore throat is an extremely common symptom that prompts millions of individuals to visit their health care provider each year. Although, the vast majority of pharyngitis cases are caused by viruses, Streptococcus pyogenes (group A Streptococcus [GAS]) is the most common bacterial cause of pharyngitis, accounting for 11,000 to 24,000 cases of invasive group A strep and several million cases of non-invasive group A strep illnesses each year in the United States. (1) 20 to 30% of pharyngitis cases in children and approximately 5% to 15% of pharyngitis infections in adults are caused by group A strep. (2)

Streptococcus pyogenes is a common commensal organism. This pathogen can coexist with its host in the naso- and oropharynx for the majority of its life, seldom inducing any clinical symptoms. The severity of Group A Streptococcus illnesses varies from asymptomatic and pharyngitis to more serious consequential complications that include sinus infections, ear infections, rheumatic fever, and post-streptococcal glomerulonephritis. (3)

Although GAS has traditionally been thought to be universally susceptible to penicillin, in 2019, two related Streptococcus pyogenes strains with reduced susceptibility to ampicillin, amoxicillin, and cefotaxime, antibiotics were presented. These strains had the same mutation in the pbp2x gene encoding penicillin-binding protein 2X (PBP2X). (4) Compelled with this information, researchers at Houston Methodist Research Institute collaborated with nearly a dozen international institutions to investigate their genome sequence library derived from 7,025 group A Streptococcus strains. Alarmingly, through bioinformatics analysis, they were able to identify 137 strains with the gene mutation of interest, many of which also showed decreased susceptibility in vitro to multiple beta-lactam antibiotics. (5)

These distressing findings may signal a public health catastrophe that is in great need of a vaccine. Recently there have been breakthroughs, and several vaccine candidates are in trials, but making a vaccine available has remained a distant goal for decades. (6)

In this era of rising antibiotic resistance, successfully identifying bacterial pharyngitis and knowing when to test for its more unusual causes is a crucial skill for any health care provider; to prevent both unnecessary treatment and the morbidity associated with missed or improperly treated infection. Rheumatic heart disease and kidney failure (glomerulonephritis) are serious sequelae of GAS infections that can be easily avoided with proper diagnosis and treatment.

In assisting health care professionals in the timely distinction between streptococcal pharyngitis and other causes, a variety of laboratory tests to detect group A strep are available, ranging from throat cultures to direct antigen detection and direct molecular assays. In spite of the clinical presentation (see photo below), the CDC maintains that throat cultures are essential for diagnosis. Although they require a longer wait time, throat cultures have high sensitivity compared to some rapid antigen tests and a lower cost than molecular methods.

Hardy Diagnostics now offers HardyCHROM™ Group A Strep agar (Cat.No. G337), a chromogenic medium for the selective cultivation and differential isolation of Group A Streptococcus. If present, Streptococcus pyogenes will grow red colonies against the white background of the media (pictured above). The unique chromogenic mixture creates a color reaction in target strains to differentiate group A strep isolates from other common streptococcus species and respiratory flora. The use of chromogenic media for pharyngitis in laboratories is of value to easily differentiate and segregate suspected positive cultures, thus eliminating the need for further confirmatory testing.

By Yesenia Morales

Microbiologist - Technical Support

HARDY DIAGNOSTICS

Ref: 1, 2, 3, 4, 5, 6

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