# Presentation: 182284

## Investigation of Contacts of Persons with Infectious Tuberculosis, 2005

- Division of Tuberculosis Elimination
- Centers for Disease Control and Prevention

*National Tuberculosis Controllers Association*
*Centers for Disease Control and Prevention*

## Background (1)

- 1962: Isoniazid (INH) demonstrated to be effective in preventing tuberculosis (TB) among household contacts of persons with TB disease
- Investigation and treatment of contacts with latent TB infection (LTBI) quickly becomes strategy in TB control and elimination in the U.S.
- 1976: American Thoracic Society (ATS) published guidelines for investigation, diagnostic evaluation, and medical treatment of TB contacts

## Background (2)

- 2005: National TB Controllers Association (NTCA) and CDC release guidelines on the investigation of contacts of persons with infectious TB
- Expanded guidelines on investigation of TB exposure and transmission, and prevention of future TB cases through contact investigations
- Standard framework for assembling information and using findings to inform decisions

## Contact Investigations – A Crucial Prevention Strategy

- On average, 10 contacts are identified for each person with infectious TB in the U.S.
- 20%–30% of all contacts have LTBI
- 1% of contacts have TB disease
- Of contacts who will ultimately have TB disease, approximately one-half develop disease in the first year after exposure

## Benefits of Contact Investigations

- Finding and treating additional TB disease cases (potentially interrupting further transmission)
- Finding and treating persons with LTBI to avert future cases

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## Contact Investigation Responsibilities

- Health departments are responsible for ensuring the conduct contact investigations
- Contact investigations are complicated activities that require
- Many interdependent decisions
- Time-consuming interventions

## Key Terms (1)

- _Case_ – A particular instance of a disease (e.g., TB). A case is detected, documented, and reported.
- _Contac_t – Someone who has been exposed to *M. tuberculosis* by sharing air space with a person with infectious TB.

## Key Terms (2)

- _Index_ – The first case or patient who comes to attention as indicator of a potential public health problem.
- _Source case or patient_ – The case or person who was the original source of infection for secondary cases or contacts; can be, but is not necessarily, the index case.

## Decisions to Initiate a Contact Investigation

## Decisions to Initiate a Contact Investigation

- Public health officials must decide which
- Contact investigations should be assigned a higher priority
- Contacts to evaluate first
- Decision to investigate an index patient depends on presence of factors used to predict likelihood of transmission

## Factors that Predict Likely Transmission of TB

- Anatomical site of the disease
- Positive sputum bacteriology
- Radiographic findings
- Behaviors that increase aerosolization of respiratory secretions
- Age
- HIV status
- Administration of effective treatment

## Characteristics of the Index Patient Associated with Increased Risk of TB Transmission

- Pulmonary, laryngeal, or pleural TB
- Acid-fast bacilli (AFB) positive sputum smear
- Cavitation on chest radiograph
- Adolescent or adult patient
- No or ineffective treatment of TB disease

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## Behaviors of the Index Patient Associated with Increased Risk of TB Transmission

- Frequent coughing
- Sneezing
- Singing
- Close social network

## Initiating a Contact Investigation (1) 

- Consider if index patient has
- Confirmed or suspected pulmonary, laryngeal, or pleural TB
- Chest radiograph consistent with pulmonary TB
- Recommended if
- Sputum smear has AFB on microscopy
- Chest radiograph indicates presence of cavities in the lung (AFB sputum smear negative)

## Initiating a Contact Investigation (2)

- Not generally indicated if
-  Sputum smear has AFB on microscopy and nucleic acid amplification (NAA) tests for *M.tuberculosis *are negative

##  Initiating a Contact Investigation (3) 

- Persons with AFB smear or culture-positive sputum and cavitary TB assigned the highest priority
- Should not be initiated for contacts who have suspected TB disease and minimal findings in support of pulmonary TB diagnosis

## Initiating a Contact Investigation (3) 

- Initiation of other investigations  depends on
- Availability of resources to be allocated
- Achievement of objectives for higher priority contact investigations

## Decision to Initiate a TB Contact Investigation

- *Acid-fast bacilli
- †Nucleic acid assay
- §Approved indication for NAA
- ¶Chest radiograph

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## Investigating the Index Patient and Sites of Transmission

## Comprehensive Index Patient Information 

- Foundation of a contact investigation
- Information to be gathered includes
- Disease characteristics
- Onset time of illness
- Names of contacts
- Exposure locations
- Current medical factors (e.g., initiation of treatment and drug susceptibility results)

## Preinterview Phase

- Collect patient background information and circumstances of illness
- Possible sources include
- Medical record
- Reporting physician
- Match patient’s name to prior TB registries and the surveillance database

## Data to Collect in Preinterview Phase (1)

- History of previous TB exposure or infection
- History of previous TB disease and treatment
- Anatomical sites of TB disease
- Symptoms of illness
- Date of onset
- Chest radiography results
- Other diagnostic imaging study results

## Data to Collect in Preinterview Phase (2)

- Histologic or bacteriologic analysis results
- Current bacteriologic results
- Anti-TB chemotherapy regimen
- HIV testing results
- Patient’s concurrent medical conditions
- Other diagnoses that may influence or impinge on the interview
- Identifying demographic information

## Determining the Infectious Period

- Focuses investigation on contacts most likely to be at risk for infection
- Sets time frame for testing contacts
- Information to assist with determining infectious period
- Approximate dates TB symptoms were noticed
- Bacteriologic results
- Extent of disease

## Start of Infectious Period

- Cannot be determined with precision; estimation is necessary
- Start is 3 months before TB diagnosis (recommended)
- Earlier start should be used in certain circumstances (e.g., patient aware of illness for longer period of time)

## Estimating the Beginning of the Infectious Period

| Characteristic of Index Case |  |  |  |
| --- | --- | --- | --- |
| TB symptoms | AFB sputum smear positive | Cavitary chest radiograph | Likely period of infectiousness |
| Yes | No | No | 3 months before symptom onset or 1st positive finding consistent with TB disease, whichever is longer |
| Yes | Yes | Yes | 3 months before symptom onset or 1st positive finding consistent with TB disease, whichever is longer |
| No | No | No | 4 weeks before date of suspected diagnosis |
| No | Yes | Yes | 3 months before positive finding consistent with TB |
| SOURCE: California Department of Health Services Tuberculosis Control Branch; California Tuberculosis Controllers Association. Contact Investigation Guidelines. Berkley, CA: California Department of Health Services; 1998. |  |  |  |

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## Closing the Infectious Period

- Effective treatment for ≥2 weeks,
- Diminished symptoms, and
- Bacteriologic response

- Infectious period closed when all the following criteria are met

## Exposure Period for Contacts

- Determined by how much time the contact spent with the index patient during the infectious period

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## Contact Investigation Interviews

- Establishing trust and rapport with patient is critical
- Interviewers should be trained in interview methods and tutored on the job
- Conducted in patient’s primary language or in conjunction with a trained interpreter
- Interviews should be conducted in person in the hospital, TB clinic, patient’s home, or a convenient location that accommodates the patients privacy

## Interviewing the Index Patient

- Minimum of two interviews should be conducted
- First interview should be conducted
- ≤1business day of reporting for infectious patients
- ≤3 business days for others
- Second interview conducted 1–2 weeks later
- Additional interviews depend on the amount of information needed and time to develop rapport with patient

## Contact Investigation Interview General Principles

- Establish rapport with patient
- Exchange information
- Review transmission settings
- Record sites of transmission
- Compile list of contacts
- Provide closure
- Conduct follow-up interviews, if needed

## Proxy Interviews

- Can build on the information provided by index patient
- Essential when patient cannot be interviewed
- Conducted with key informants most likely to know the patients’ practices, habits, and behaviors
- Jeopardizes patient confidentiality

## Field Investigation (Site Visits)

- Site visits are complementary to interviewing
- Should be made ≤3 days of the initial interview
- Elicits additional contact information; especially helpful for finding children
- Lack of site visits has contributed to TB outbreaks

## Follow-up Steps

- Continuing investigation is shaped by reassessments of ongoing results
- Notification and follow-up communication with other jurisdictions should be arranged for out-of-area contacts

## Specific Investigation Plan

- Investigation plan should include
- Information gathered in interviews and site visits
- Registry of contacts and their assigned priorities
- Written timeline for monitoring the investigation progress
- Data recorded on standardized forms
- Part of the permanent medical record

## Time Frames for Initial Follow-up of Contacts Exposed to TB

| 

Type of Contact | Business days from listing of a contact to initial encounter* | Business days from initial encounter to completion of medical evaluation† |
| --- | --- | --- |
| High priority contact: index case AFB sputum smear positive or cavitary disease on chest x-ray | 7 | 5 |
| High priority contact: index case AFB sputum smear negative§ | 7 | 10 |
| Medium priority contact: regardless of AFB sputum smear or culture result | 14 | 10 |
| *A face-to-face meeting that allows the health care worker to assess the overall health of the contact, administer a TST, and schedule further evaluation.
†The medical evaluation is complete when the contact’s status (LTBI or TB disease) is determined.
§Abnormal chest x-ray consistent with TB disease, might be NAA positive and /or AFB culture positive |  |  |
| SOURCE: California Department of Health Services Tuberculosis Control Branch; California Tuberculosis Controllers Association. Contact Investigation Guidelines. Berkley, CA: California Department of Health Services; 1998. |  |  |

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## Assigning Priorities to Contacts

## Assigning Priorities to Contacts (1)

- Priorities should be assigned to contacts and resources allocated to complete all investigative steps for high-and medium-priority contacts.
- Any contact not classified as high or medium priority is assigned a low priority.

## Assigning Priorities to Contacts (2)

- Priorities based on likelihood of infection and hazards to the contact if infected
- Priority scheme directs resources to contacts who
- Have secondary case of TB disease
- Have recent *M. tuberculosis* infection (most likely to benefit from treatment)
- Are most likely to develop TB disease if infected or could suffer severe morbidity if they develop TB disease

## Factors for Assigning Contact Priorities

- Characteristics of the index patient
- Characteristics of contacts
- Age
- Immune status
- Other medical conditions
- Exposure

## Prioritization of Contacts (1)

| Patient has pulmonary, laryngeal, or pleural TB with cavitary lesion on chest radiograph or is AFB sputum smear positive |  |
| --- | --- |
| Household contact | High |
| Contact <5 years of age | High |
| Contact with medical risk factor (HIV or other medical risk factor) | High |
| Contact with exposure during medical procedure (bronchoscopy, sputum induction, or autopsy) | High |
| Contact in a congregate setting | High |
| Contact exceeds duration/environment limits (limits per unit time established by the health department for high-priority contacts) | High |
| Contact is ≥ 5 years and ≤ 15 years of age | Medium |
| Contact exceeds duration/environment limits (limits per unit time established by the health department for medium-priority contacts) | Medium |
| Any contact not classified as high or medium priority is assigned a low priority. |  |

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## Prioritization of Contacts (2)

| Patient is a suspect or has confirmed pulmonary/pleural TB – AFB smear negative, abnormal chest radiograph consistent with TB disease, may be NAA and/or culture positive |  |
| --- | --- |
| Contact <5 years of age | High |
| Contact with medical risk factor (e.g., HIV) | High |
| Contact with exposure during medical procedure (bronchoscopy, sputum induction, or autopsy) | High |
| Household contact | Medium |
| Contact exposed in congregate setting | Medium |
| Contact exceeds duration/environment limits (limits per unit time established by the local TB control program) | Medium |
| Any contact not classified as high or medium priority is assigned a low priority. |  |

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## Prioritization of Contacts (3)

| Patient is a suspect pulmonary TB case – AFB smear negative, NAA negative/culture negative, abnormal chest radiograph not consistent with TB disease |  |
| --- | --- |
| Household contact | Medium |
| Contact <5 years of age | Medium |
| Contact with medical risk factor (e.g., HIV infection or other immunocompromising condition) | Medium |
| Contact with exposure during medical procedure (bronchoscopy, sputum induction, or autopsy) | Medium |
| Any contact not classified as high or medium priority is assigned a low priority. |  |

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## Diagnostic and Public Health Evaluation of Contacts

## Initial Assessment of Contacts

- Should be accomplished within 3 working days of the contact having been listed in the investigation
- Gathers background health information
- Permits face-to-face assessment of person’s health

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## Information to Collect During Initial Assessment (1)

- Previous *M. tuberculosis* infection or disease and related treatment
- Contact’s verbal report and documentation of previous TST results
- Current symptoms of TB illness

## Information to Collect During Initial Assessment (2)

- Medical conditions making TB disease more likely
- Mental health disorders
- Type, duration, and intensity of TB exposure
- Sociodemographic factors

## Information to Collect During Initial Assessment (3)

- HIV status; contacts should be offered HIV counseling and testing if status unknown
- Information regarding social, emotional, and practical matters that might hinder participation

## Reassess Strategy After Initial Information Collected

- After initial information collected
- Priority assignments should be reassessed
- Medical plan for diagnostic tests and possible treatment can be formulated for high- and medium-priority contacts

## Tuberculin Skin Testing

- All high or medium priority contacts who do not have a documented previous positive tuberculin skin test (TST) or previous TB disease should receive a TST at the initial encounter.
- If not possible, TST should be administered
- ≤7 working days of listing high-priority contacts
- ≤14 days of listing medium-priority contacts

## Interpreting Skin Test Reaction

- ≥5 mm induration is positive for any contact
- Two-step procedure should not be used for testing contacts
- A contact whose second TST is positive after initial negative result should be classified as recently infected

## Postexposure Tuberculin Skin Testing

- Window period is 8–10 weeks after exposure ends
- Contacts who have a positive result after a previous negative result are said to have had a change in tuberculin status from negative to positive

## Medical Evaluation

- All contacts whose skin test reaction induration is ≥5 mm or who report any symptoms consistent with TB disease should undergo further examination and testing for TB

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## Evaluation and Follow-up of Children <5 Years of Age

- Always assigned a high priority as contacts
- Should receive full diagnostic medical evaluation, including a chest radiograph
- If TST ≤5 mm of induration and last exposure <8 weeks, LTBI treatment recommended (after TB disease excluded)
- Second TST 8–10 weeks after exposure; decision to treat is reconsidered
- Negative TST – treatment discontinued
- Positive TST – treatment continued

## Evaluation and Follow-up of Immunosuppressed Contacts

- Should receive full diagnostic medical evaluation, including a chest radiograph
- If TST negative ≥8 weeks after end of exposure, full course of treatment for LTBI recommended (after TB disease is excluded)

## Medical Treatment for Contacts with LTBI

## Health Department Responsibilities 

- Focusing resources on contacts in most need of treatment
- Monitoring treatment, including that of contacts who receive care outside the health department
- Providing directly observed therapy (DOT), incentives, and enablers

## Window-Period Prophylaxis

- The frequency, duration, and intensity of exposure
- Corroborative evidence of transmission from the index patient

- Decision to treat contacts with a negative skin test result should take the following factors into consideration

## Prophylactic Treatment

- With HIV infection
- Taking immunosuppressive therapy for organ transplant
- Taking anti-tumor necrosis factor alpha (TNF-α) agents

- Prophylactic treatment (after TB disease is excluded) of presumed *M. tuberculosis* infection recommended for persons

## Treatment After Exposure to Drug-Resistant TB

- Consultation with physician with MDR expertise recommended for selecting a LTBI regimen
- Contacts should be monitored for 2 years after exposure

## Selecting Contacts for Directly Observed Therapy

- Contacts aged <5 years
- Contacts who are HIV infected or otherwise substantially immunocompromised
- Contacts with a change in their tuberculin skin test status from negative to positive
- Contacts who might not complete treatment because of social or behavior impediments

## When to Expand a Contact Investigation

## Determining When to Expand a Contact Investigation

- Consideration of the following factors recommended
- Achievement of program objectives with high- and medium-priority contacts
- Extent of recent transmission
- Unexpectedly large rate of infection or TB disease in high-priority contacts
- Evidence of second-generation transmission
- TB disease in any contacts who had been assigned low priority
- Infection in any contacts aged <5 years
- Contacts with change in skin test status from negative to positive

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## Strategy for Expanding a Contact Investigation

- Should be based on the investigation data
- Results should be reviewed weekly
- In absence of recent transmission, investigation should not be expanded to lower-priority groups

## Communicating Through the News Media

## Possible Situations for News Coverage

- Certain contact investigations have the potential for sensational news coverage. Examples include
- Involving numerous contacts (especially children)
- Occurring in public settings
- Occurring in workplaces
- Associated with TB fatalities
- Associated with drug-resistant TB

## Reasons for Participating in News Media Coverage (1)

- Educates the public regarding the nature of TB
- Reminds public of continued presence of TB
- Provides a complementary method to alert exposed contacts of the need for seeking medical evaluation
- Relieves unfounded public fears regarding TB

## Reasons for Participating in News Media Coverage (2)

- Illustrates the health department’s leadership in communicable disease control
- Ensures that constructive public inquiries are directed to the health department
- Validates the need for public resources to be directed to disease control

## Potential Drawbacks to News Coverage

- Increase public anxiety
- Cause unexposed person seeking unnecessary medical care
- Contribute to unfavorable views of the health department
- Contribute to spread of misinformation
- Trigger unconstructive public inquiries
- Unintended disclosure of confidential information

## Strategy for News Coverage

- Anticipatory preparation of clear media messages is recommended
- Develop communication objectives
- Issue news release in advance of any other media coverage
- Collaborate with partners outside of the health department

## Data Management and Evaluation of Contact Investigations

## Data Collection

- Management of care and follow-up index patients and contacts
- Epidemiologic analysis of investigation in progress and investigations overall
- Program evaluation using performance indicators that reflect performance objectives

- Three broad purposes in contact investigations

## Reasons Contact Investigation Data are Needed

- Presents broad amount of demographic, epidemiologic, historic, and medical information needed to provide comprehensive care
- Provides information on process steps necessary for monitoring timeline objectives
- Provides information needed to reassess investigation strategy

## Confidentiality and Consent in Contact Investigations

## Safeguarding Confidentiality	

- Challenging and difficult during contact investigations
- Essential to maintaining credibility and trust
- Constant attention required to maintain confidentiality
- Specific policies for release of confidential information related to contact investigations are recommended

## Confidentiality and Consent

- Contact investigation policies and training
- Informed consent
- Site investigations
- Other medical conditions besides TB

- TB control programs should address the following confidentiality and consent issues before initiation of contact investigations

## Staffing and Training for Contact Investigations

## Staffing and Training for Contact Investigations

- Contact investigations involve personnel in the health department and other health care delivery systems
- Contact investigation tasks require multiple functions and skills
- Training is essential for successful contact investigations

## Contact Investigations in Special Circumstances

## Definition of an Outbreak

- During (and because of) a contact investigation, 2 or more contacts are found to have active TB, regardless of their assigned priority; or
- Any 2 or more cases occurring within a year of each other, discovered to be linked, and the linkage is established outside of a contact investigation

## TB Outbreaks

- A TB patient was contagious
- Contacts were exposed for a substantial period of time
- The interval since exposure has been sufficient for infection to progress to disease (interval may be shortened by HIV infection)

- A TB outbreak is a sign of extensive transmission and implies that

## Develop Outbreak Strategy Based on Risk Factors

- Contagious TB undiagnosed or untreated for an extended period, or an extremely contagious case
- Source patient visiting multiple sites
- Patient and contacts in close or prolonged company
- Environment promoting transmission
- Contacts very susceptible to disease after   *M. tuberculosis* infection
- Gaps in contact investigations and follow-up
- Extra-virulent strain of *M.tuberculosis*

## Congregate Settings

- Substantial number of contacts
- Incomplete information regarding contact names and locations
- Incomplete data for determining priorities
- Difficulty in maintaining confidentiality
- Collaboration with officials and administrators who are unfamiliar with TB
- Legal implications
- Media coverage

- Concerns associated with congregate settings

## Congregate Settings – Designating Priorities

- Site specific
- Customized algorithm required for each situation
- Source-case characteristics
- Duration and proximity of exposure
- Environmental factors that modify transmission
- Susceptibility of contacts

## Congregate Settings - Setting-Based Investigation

- Interview and test contacts on site is optimum approach
- Alternative is evaluation at the health department with additional personnel and extended hours
- As last resort, notify contacts in writing to seek diagnostic evaluation with their own health care provider

## Correctional Facilities

- Establish preexisting formal collaboration between correctional and public health officials
- Trace high-priority contacts who are transferred, released, or paroled before medical evaluation for TB
- Low completion rate is anticipated unless follow-through supervision can be arranged for released or paroled inmates

## Workplaces

- Duration and proximity of exposure can be greater than for other settings
- Details to gather from index patient during initial interview include
- Employment hours
- Working conditions
- Workplace contacts
- Occasional customers of workplace should be designated as low priority

## Hospitals and Other Health-Care Settings

- Personnel collaborating with hospitals and other health-care agencies should have knowledge of legal requirements
- Plan investigation jointly with health department and setting (division of responsibilities)
- Majority of health-care settings have policies for testing employees for *M. tuberculosis* infection

## Schools

- Early collaboration with school officials and community members is recommended
- Issues of consent, assent, and disclosure of information more complex for minors
- Site visits should be conducted to check indoor spaces, observe general conditions, and interview maintenance personnel regarding ventilation

## Shelters and Other Settings Providing Services for Homeless Persons

- Challenges include
- Locating the patient and contacts if mobile
- Episodic incarceration
- Migration from one jurisdiction to another
- Psychiatric illnesses
- Preexisting medical conditions
- Site visits and interviews are crucial
- Work with setting administrators to offer onsite supervised intermittent treatment

## Interjurisdictional Contact Investigations

- Requires joint strategies for finding contacts, having them evaluated, treating infected contacts, and gathering data
- Health department that counts index patient is responsible for leading the investigation and notifying health departments in other jurisdictions

## Source-Case Investigations

## Source-Case Investigations

- Seeks the source of recent *M.tuberculosis* infection
- In the absence of cavitary disease, young children usually do not transmit *M.tuberculosis* to others
- Recommended only when TB control program is achieving its objectives when investigating infectious cases

## Child with TB Disease

- Source-case investigations considered for children <5 years of age
- May be started before diagnosis of TB confirmed

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## Child with LTBI

- Search for source of infection for child is unlikely to be productive
- Recommended only with infected children <2 years of age, and only if data are monitored to determine the value of the investigation

## Procedures for Source-Case Investigation

- Same procedure as standard contact investigation
- Patient or guardians best informants (associates)
- Focus on associates who have symptoms of TB disease
- Should begin with closest associates

## Data collection

- Number of index patients investigated for their sources
- Number of associates screened for TB disease
- Number of times a source is found

- Data needed for assessing the productivity of source-case investigations

## Cultural Competency and Social Network Analysis

## Cultural Competence

- Knowledge and interpersonal skills that allow health-care providers to appreciate and work with persons from cultures other than their own
- Ability to understand cultural norms and to bridge gaps requires training and experience

## Social Network Analysis

- Social Network **–** linkage of persons and places where *M. tuberculosis* is spread via shared air space
- Social Network Analysis **–** methodology of visualizing and quantitating the relative importance of members in a social network
- Social Network Analysis assumes there is some detectable patterning of the TB cases and their contacts in a community

## Personal Networks for Two TB Cases

**Ted**

**Ali**

**Moe**

**Rita**

## Combined: A Social Network

**Ali**

**Moe**

**Rita**

- Allows review of multiple rather than individual personal networks

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## Combined: A Social Network with Place

**Ali**

**Moe**

**Rita**

**Mel’s Bar**

## Social Network Analysis – Approach (1)

- Provides a systematic method to deal with data already gathered in routine contact investigations
- Analysis of the network can help identify important contacts (i.e., those most likely to be infected)
- Real-time monitoring of network growth may facilitate early detection of outbreaks

## Social Network Analysis – Approach (2)

- May help programs focus control efforts
- May offer effective way to list contacts and assign priorities
- Has been tested retrospectively on TB outbreak and contact investigations

## Reference

- Guidelines for the Investigation of Contacts of Persons with Infectious Tuberculosis: Recommendations from the National Tuberculosis Controllers Association and CDC. *MMWR *2005; 54 (No. RR–15)
-  http://www.cdc.gov/mmwr/pdf/rr/rr5415.pdf

## Continuing Education Credits (1)

- Participants will be able to receive one of the following
- Continuing Medical Education (CME) credit – 2.75 hours
- Continuing Nursing Education (CNE) credit – 3.2 hours
- Continuing Education Unit (CEU) – 0.25 hours
- Certified Health Education Specialist (CHES) credit – 2.5 hours
- Participants are required to read and study the guidelines, take a test, and complete an evaluation.

## Continuing Education Credits (2)

- MMWR CE Credit http://www.cdc.gov/mmwr/cme/conted.html
- Continuing education credits will be available until December 16, 2008

## Additional Resources

- For additional information on TB, visit the CDC Division of Tuberculosis Elimination Website at http://www.cdc.gov/tb
- Guidelines Available Online
- CDC’s Morbidity and Mortality Weekly Report - http://www.cdc.gov/mmwr