Recognition of microbial viability via TLR8 drives TFH cell differentiation and vaccine responses.

Live attenuated vaccines are generally highly efficacious and often superior to inactivated vaccines, yet the underlying mechanisms of this remain largely unclear. Here we identify recognition of microbial viability as a potent stimulus for follicular helper T cell (TFH cell) differentiation and vaccine responses. Antigen-presenting cells (APCs) distinguished viable bacteria from dead bacteria through Toll-like receptor 8 (TLR8)-dependent detection of bacterial RNA. In contrast to dead bacteria and other TLR ligands, live bacteria, bacterial RNA and synthetic TLR8 agonists induced a specific cytokine profile in human and porcine APCs, thereby promoting TFH cell differentiation. In domestic pigs, immunization with a live bacterial vaccine induced robust TFH cell and antibody responses, but immunization with its heat-killed counterpart did not. Finally, a hypermorphic TLR8 polymorphism was associated with protective immunity elicited by vaccination with bacillus Calmette-Guérin (BCG) in a human cohort. We have thus identified TLR8 as an important driver of TFH cell differentiation and a promising target for TFH cell-skewing vaccine adjuvants.

Old vaccines for new infections: Exploiting innate immunity to control COVID-19 and prevent future pandemics.

The COVID-19 pandemic triggered an unparalleled pursuit of vaccines to induce specific adaptive immunity, based on virus-neutralizing antibodies and T cell responses. Although several vaccines have been developed just a year after SARS-CoV-2 emerged in late 2019, global deployment will take months or even years. Meanwhile, the virus continues to take a severe toll on human life and exact substantial economic costs. Innate immunity is fundamental to mammalian host defense capacity to combat infections. Innate immune responses, triggered by a family of pattern recognition receptors, induce interferons and other cytokines and activate both myeloid and lymphoid immune cells to provide protection against a wide range of pathogens. Epidemiological and biological evidence suggests that the live-attenuated vaccines (LAV) targeting tuberculosis, measles, and polio induce protective innate immunity by a newly described form of immunological memory termed "trained immunity." An LAV designed to induce adaptive immunity targeting a particular pathogen may also induce innate immunity that mitigates other infectious diseases, including COVID-19, as well as future pandemic threats. Deployment of existing LAVs early in pandemics could complement the development of specific vaccines, bridging the protection gap until specific vaccines arrive. The broad protection induced by LAVs would not be compromised by potential antigenic drift (immune escape) that can render viruses resistant to specific vaccines. LAVs might offer an essential tool to "bend the pandemic curve," averting the exhaustion of public health resources and preventing needless deaths and may also have therapeutic benefits if used for postexposure prophylaxis of disease.

The impact of BCG dose and revaccination on trained immunity.

The innate immune system can display heterologous memory-like responses termed trained immunity after stimulation by certain vaccinations or infections. In this randomized, placebo-controlled trial, we investigated the modulation of Bacille Calmette-Guérin (BCG)-induced trained immunity by BCG revaccination or high-dose BCG administration, in comparison to a standard dose. We show that monocytes from all groups of BCG-vaccinated individuals exerted increased TNFα production after ex-vivo stimulation with various unrelated pathogens. Similarly, we observed increased amounts of T-cell-derived IFNγ after M. tuberculosis exposure, regardless of the BCG intervention. NK cell cytokine production, especially after heterologous stimulation with the fungal pathogen Candida albicans, was predominantly boosted after high dose BCG administration. Cytokine production capacity before vaccination was inversely correlated with trained immunity. While the induction of a trained immunity profile is largely dose- or frequency independent, baseline cytokine production capacity is associated with the magnitude of the innate immune memory response after BCG vaccination.

The Effect of a Second Dose of Measles Vaccine at 18 Months of Age on Nonaccidental Deaths and Hospital Admissions in Guinea-Bissau: Interim Analysis of a Randomized Controlled Trial.

BACKGROUND: The world is set on the eradication of measles. Continuation of the measles vaccine (MV) after eradication could still reduce morbidity because the MV has so-called beneficial nonspecific effects. We evaluated the effect of a "booster" dose of the MV on overall severe morbidity. METHODS: We conducted a randomized controlled trial among children aged 17.5 to 48 months in Guinea-Bissau, where the MV is recommended only at 9 months of age. At the time of this interim analysis, 3164 children had been allocated 1:1 to a second dose of measles vaccine (MV2) at 18 months of age or to no vaccine. Severe morbidity (a composite outcome of nonaccidental deaths and hospital admissions) rate ratios (SMRRs) were calculated by Cox regression analysis censored for national oral polio vaccine (OPV) campaigns. RESULTS: There were no measles cases during the trial period. There were 43 nonaccidental deaths or hospital admissions during follow-up. Severe morbidity was 2.6 per 100 person-years in the MV2 group and 3.6 per 100 person-years among controls; hence, the estimated effect of MV2 on severe morbidity was 28% (SMRR, 0.72; 95% confidence interval [CI], .38-1.38). At 12 months of follow-up, the number needed to treat to prevent 1 severe morbidity event was 137 children. After OPV campaigns, the estimated effect of MV2 was reduced to 9% (SMRR, 0.91; 95% CI, .46-1.81). CONCLUSIONS: MV2 may reduce nonmeasles severe morbidity by 28% (-38% to 62%), although this did not achieve statistical significance in this study. If significant in higher powered studies, this has major implications for child health, even after measles eradication. CLINICAL TRIALS REGISTRATION: NCT02943681.

Interacting, Nonspecific, Immunological Effects of Bacille Calmette-Guérin and Tetanus-diphtheria-pertussis Inactivated Polio Vaccinations: An Explorative, Randomized Trial.

BACKGROUND: Certain vaccines, such as Bacille Calmette-Guérin (BCG), have nonspecific effects, which modulate innate immune responses and lead to protection against mortality from unrelated infections (trained immunity). In contrast, in spite of the disease-specific effects, an enhanced overall mortality has been described after diphtheria-tetanus-pertussis (DTP) vaccination in females. This randomized trial aimed to investigate the nonspecific immunological effects of BCG and DTP-containing vaccines on the immune response to unrelated pathogens. METHODS: We randomized 75 healthy, female, adult volunteers to receive either BCG, followed by a booster dose of tetanus-diphtheria-pertussis inactivated polio vaccine (Tdap) 3 months later; BCG and Tdap combined; or Tdap followed by BCG 3 months later. Blood was collected before vaccination, as well as at 1 day, 4 days, 2 weeks, and 3 months after the first vaccination(s), plus 2 weeks after the second vaccination. Ex vivo leukocyte responses to unrelated stimuli and pathogens were assessed. RESULTS: Tdap vaccination led to short-term potentiation and long-term repression of monocyte-derived cytokine responses, and short-term as well as long-term repression of T-cell reactivity to unrelated pathogens. BCG led to short-term and long-term potentiation of monocyte-derived cytokine responses. When given together with Tdap or after Tdap, BCG abrogated the immunosuppressive effects of Tdap vaccination. CONCLUSIONS: Tdap induces immunotolerance to unrelated antigens, which is partially restored by concurrent or subsequent BCG vaccination. These data indicate that the modulation of heterologous immune responses is induced by vaccination with Tdap and BCG, and more studies are warranted to investigate whether this is involved in the nonspecific effects of vaccines on mortality. CLINICAL TRIALS REGISTRATION: NCT02771782.

Differential effects of BCG vaccine on immune responses induced by vi polysaccharide typhoid fever vaccination: an explorative randomized trial.

The Vi polysaccharide typhoid fever vaccine (TFV) provides incomplete protection against typhoid fever. BCG, the vaccine against tuberculosis, can potentiate immune responses to other vaccines through induction of trained innate immunity and heterologous adaptive immunity. We performed an explorative, randomized, noncontrolled open trial to investigate whether BCG vaccination increases humoral and cellular response to TFV and whether BCG and TFV modulate nonspecific immune responses. Thirty volunteers were randomized to receive either TFV alone or BCG followed by TFV after 2 weeks. Ex vivo leukocyte responses and anti-Vi IgG antibody titers were measured 2 weeks and 3 months after TFV. BCG administration prior to TFV vaccination did not increase specific humoral or cellular immune responses to Salmonella typhi. TFV vaccination decreased pro-inflammatory responses to non-related stimuli. This effect was counteracted by prior BCG administration, which also led to decreased IL-10 and increased IL-22 responses to non-related stimuli. In an in vitro model of trained immunity TFV led to immunotolerance, which was partially reversed by BCG-induced trained immunity. BCG does not modulate adaptive immune responses to TFV but partially prevents inhibition of innate immune responses induced by TFV. Nonspecific effects of vaccines to unrelated microbial stimuli must be considered in the evaluation of their biological effects (ClinicalTrials.gov NCT02175420).

How are children who are delayed in the Childhood Vaccination Programme vaccinated: A nationwide register-based cohort study of Danish children aged 15-24 months and semi-structured interviews with vaccination providers.

Aims: Delay of childhood vaccinations is common and influences efforts to reduce targeted diseases. In Denmark, the diphtheria, tetanus, pertussis, polio and Haemophilus influenzae type b (DTaP-IPV-Hib) vaccine is recommended at ages 3, 5 and 12 months and the first measles-mumps-rubella vaccine (MMR-1) at 15 months. Following guidelines, children delayed at age 15 months should receive MMR-1 and DTaP-IPV-Hib-3 simultaneously, unless DTaP-IPV-Hib-2 was received less than 6 months ago, when MMR-1 alone is recommended. We studied compliance with these guidelines and the reasons for non-compliance with a focus on vaccination providers. Methods: We used a nationwide register-based cohort study of children born in Denmark between January 2000 and June 2013, who were lacking MMR-1 and DTaP-IPV-Hib-3 at age 15 months and were followed to 24 months. We also performed semi-structured telephone interviews with vaccination providers. Results: The study consisted of 156,921 children (18% of the children born in the period). Among the 40,060 children who had received DTaP-IPV-Hib-2 less than 6 months ago, 37,892 (95%) received MMR-1 alone. Among the 88,469 children who had received DTaP-IPV-Hib-2 more than 6 months ago, 6334 (7%) received DTaP-IPV-Hib-3 and MMR-1 simultaneously. The interviews indicated that some vaccination providers are reluctant to give multiple vaccinations at the same visit and some have a preference of following the usual sequence in the programme. Conclusions: Vaccination providers generally complied with the recommended minimum 6 months' interval between DTaP-IPV-Hib-2 and DTaP-IPV-Hib-3. Conversely, there was a low compliance with the recommendation to administer DTaP-IPV-Hib-3 and MMR-1 simultaneously. More efforts are needed to ensure timely vaccination.

Revaccination With Measles-Mumps-Rubella Vaccine and Infectious Disease Morbidity: A Danish Register-based Cohort Study.

Background: It has been hypothesized that revaccination with live vaccines is associated with reductions in off-target morbidity and mortality. We examined if revaccination with the live measles, mumps, and rubella vaccine (MMR) is associated with a lower rate of off-target infections. Methods: We performed a register-based nationwide cohort study that included 295559 children born in Denmark from April 2004 to December 2010. The cohort were followed from age 47 months (1 month before turning age 4 years, which is the recommended age of the second MMR [MMR-2]) until age 60 months. In Cox regression, we estimated adjusted incidence rate ratios (aIRRs) of antibiotic prescriptions and hospital admissions for any infection comparing MMR-2 as most recent vaccine with not having MMR-2 as the most recent vaccine. Results: There was no association between MMR-2 and antibiotic prescriptions (aIRR, 1.01; 95% confidence interval [CI], 0.99-1.02). The aIRR for the association between MMR-2 and admissions for infection of any duration was 0.93 (95% CI, 0.88-0.98). For admissions for infection lasting 0 to 1 day, the aIRR was 0.97 (95% CI, 0.90-1.03) compared with the aIRR of 0.84 (95% CI, 0.74-0.95) for admissions for infection lasting 2 days or longer (test for equality of aIRRs, P = .039). Conclusions: In this study, revaccination with MMR appeared safe in relation to off-target infections and was associated with a lower rate of severe off-target infections. More studies of the possible association between revaccination with live attenuated vaccines and off-target infections are needed.

Opposite effects of Vaccinia and modified Vaccinia Ankara on trained immunity.

Vaccines such as Vaccinia or BCG have non-specific effects conferring protection against other diseases than their target infection, which are likely partly mediated through induction of innate immune memory (trained immunity). MVA85A, a recombinant strain of modified Vaccinia Ankara (MVA), has been suggested as an alternative vaccine against tuberculosis, but its capacity to induce positive or negative non-specific immune effects has not been studied. This study assesses whether Vaccinia and MVA are able to induce trained innate immunity in monocytes. Human primary monocytes were primed in an in vitro model with Vaccinia or MVA for 1 day, after which the stimulus was washed off and the cells were rechallenged with unrelated microbial ligands after 1 week. Heterologous cytokine responses were assessed and the capacity of MVA to induce epigenetic changes at the level of cytokine genes was investigated using chromatin immunoprecipitation and pharmacological inhibitors. Monocytes trained with Vaccinia showed significantly increased IL-6 and TNF-α production to stimulation with non-related stimuli, compared to non-trained monocytes. In contrast, monocytes primed with MVA showed significant decreased heterologous IL-6 and TNF-α responses, an effect which was abrogated by the addition of a histone methyltransferase inhibitor. No effects on H3K4me3 were observed after priming with MVA. It can be thus concluded that Vaccinia induces trained immunity in vitro, whereas MVA induces innate immune tolerance. This suggests the induction of trained immunity as an immunological mechanism involved in the non-specific effects of Vaccinia vaccination and points to a possible explanation for the lack of effect of MVA85A against tuberculosis.

A Two-Center Randomized Trial of an Additional Early Dose of Measles Vaccine: Effects on Mortality and Measles Antibody Levels.

Background: In addition to protecting against measles, measles vaccine (MV) may have beneficial nonspecific effects. We tested the effect of an additional early MV on mortality and measles antibody levels. Methods: Children aged 4-7 months at rural health and demographic surveillance sites in Burkina Faso and Guinea-Bissau were randomized 1:1 to an extra early standard dose of MV (Edmonston-Zagreb strain) or no extra MV 4 weeks after the third diphtheria-tetanus-pertussis-hepatitis B-Haemophilus influenzae type b vaccine. All children received routine MV at 9 months. We assessed mortality through home visits and compared mortality from enrollment to age 3 years using Cox proportional hazards models, censoring for subsequent nontrial MV. Subgroups of participants had blood sampled to assess measles antibody levels. Results: Among 8309 children enrolled from 18 July 2012 to 3 December 2015, we registered 145 deaths (mortality rate: 16/1000 person-years). The mortality was lower than anticipated and did not differ by randomization group (hazard ratio, 1.05; 95% confidence interval, 0.75-1.46). At enrollment, 4% (16/447) of children in Burkina Faso and 21% (90/422) in Guinea-Bissau had protective measles antibody levels. By age 9 months, no measles-unvaccinated/-unexposed child had protective levels, while 92% (306/333) of early MV recipients had protective levels. At final follow-up, 98% (186/189) in the early MV group and 97% (196/202) in the control group had protective levels. Conclusions: Early MV did not reduce all-cause mortality. Most children were susceptible to measles infection at age 4-7 months and responded with high antibody levels to early MV. Clinical Trials Registration: NCT01644721.

A small jab - a big effect: nonspecific immunomodulation by vaccines.

Recent epidemiological studies have shown that, in addition to disease-specific effects, vaccines against infectious diseases have nonspecific effects on the ability of the immune system to handle other pathogens. For instance, in randomized trials tuberculosis and measles vaccines are associated with a substantial reduction in overall child mortality, which cannot be explained by prevention of the target disease. New research suggests that the nonspecific effects of vaccines are related to cross-reactivity of the adaptive immune system with unrelated pathogens, and to training of the innate immune system through epigenetic reprogramming. Hence, epidemiological findings are backed by immunological data. This generates a new understanding of the immune system and about how it can be modulated by vaccines to impact the general resistance to disease.

Nonspecific effect of BCG vaccination at birth on early childhood infections: a randomized, clinical multicenter trial.

BACKGROUND: Childhood infections are common and Bacillus Calmette-Guérin (BCG) vaccination at birth may prevent these via nonspecific effects. METHODS: A randomized, clinical multicenter trial. All women planning to give birth (n = 16,521) at the three study sites were invited during the recruitment period. Participating children were randomized to receive BCG within 7 d of birth or to a no intervention control group. Parent-reported infections (events) were collected using telephone interviews at 3 and 13 mo. Data collectors were blinded to allocation. RESULTS: The analyses included 4,224/4,262 (99%) and 4,192/4,262 (98%) children at 3 and 13 mo. From 0 to 3 mo, there were 291 events in the BCG group vs. 336 events in the control group, incidence rate ratio (IRR) = 0.87 (95% confidence interval (CI): 0.72 to 1.05). In this age group, the IRR was 0.62 (95% CI: 0.39 to 0.98) if the mother was BCG vaccinated. From 3 to 13 mo, there were 7,028 vs. 6,791 events, IRR = 1.02 (95% CI: 0.97 to 1.07). CONCLUSION: This study did not find a nonspecific public health benefit of BCG on parent-reported infections. BCG may have reduced the incidence of infections in children of BCG-vaccinated mothers during the first 3 mo.

Measles virus antibody responses in children randomly assigned to receive standard-titer edmonston-zagreb measles vaccine at 4.5 and 9 months of age, 9 months of age, or 9 and 18 months of age.

The World Health Organization recommends administration of measles vaccine (MV) at age 9 months in low-income countries. We tested the measles virus antibody response at 4.5, 9, 18, and 24 months of age for children randomly assigned to receive standard-titer Edmonston-Zagreb MV at 4.5 and 9 months, at 9 months, or at 9 and 18 months of age. At 4.5 months of age, 75% had nonprotective measles virus antibody levels. Following receipt of MV at 4.5 months of age, 77% (316/408) had protective antibody levels at 9 months of age; after a second dose at 9 months of age, 97% (326/337) had protective levels at 24 months of age. In addition, the response at both 9 and 24 months of age was inversely correlated with the antibody level at receipt of the first dose of MV, and the second dose of MV, received at 9 months of age, provided a significant boost in antibody level to children who had low antibody levels. In the group of 318 children who received MV at 9 months of age, with or without a second dose at 18 months of age, 99% (314) had protective levels at 24 months of age. The geometric mean titer at 24 months of age was significantly lower in the group that received MV at 4.5 and 9 months of age than in the group that received MV at 9 months of age (P = .0001). In conclusion, an early 2-dose MV schedule was associated with protective measles virus antibody levels at 24 months of age in nearly all children. Clinical Trials Registration. NCT00168558.

A randomized trial of a standard dose of Edmonston-Zagreb measles vaccine given at 4.5 months of age: effect on total hospital admissions.

Observational studies and trials from low-income countries indicate that measles vaccine has beneficial nonspecific effects, protecting against non-measles-related mortality. It is not known whether measles vaccine protects against hospital admissions. Between 2003 and 2007, 6417 children who had received the third dose of diphtheria, tetanus, and pertussis vaccine were randomly assigned to receive measles vaccine at 4.5 months or no measles vaccine; all children were offered measles vaccine at 9 months of age. Using hospital admission data from the national pediatric ward in Bissau, Guinea-Bissau, we compared admission rates between enrollment and the 9-month vaccination in Cox models, providing admission hazard rate ratios (HRRs) for measles vaccine versus no measles vaccine. All analyses were conducted stratified by sex and reception of neonatal vitamin A supplementation (NVAS). Before enrollment the 2 groups had similar admission rates. Following enrollment, the measles vaccine group had an admission HRR of 0.70 (95% confidence interval [CI], .52-.95), with a ratio of 0.53 (95% CI, .32-.86) for girls and 0.86 (95% CI, .58-1.26) for boys. For children who had not received NVAS, the admission HRR was 0.53 (95% CI, .34-.84), with an effect of 0.30 (95% CI, .13-.70) for girls and 0.73 (95% CI, .42-1.28) for boys (P = .08, interaction test). The reduction in admissions was separately significant for measles infection (admission HRR, 0 [95% CI, 0-.24]) and respiratory infections (admission HRR, 0.37 [95% CI, .16-.89]). Early measles vaccine may have major benefits for infant morbidity patterns and healthcare costs. Clinical trials registration NCT00168558.

Measles vaccination in the presence or absence of maternal measles antibody: impact on child survival.

BACKGROUND: Measles vaccine (MV) has a greater effect on child survival when administered in early infancy, when maternal antibody may still be present. METHODS: To test whether MV has a greater effect on overall survival if given in the presence of maternal measles antibody, we reanalyzed data from 2 previously published randomized trials of a 2-dose schedule with MV given at 4-6 months and at 9 months of age. In both trials antibody levels had been measured before early measles vaccination. RESULTS: In trial I (1993-1995), the mortality rate was 0.0 per 1000 person-years among children vaccinated with MV in the presence of maternal antibody and 32.3 per 1000 person-years without maternal antibody (mortality rate ratio [MRR], 0.0; 95% confidence interval [CI], 0-.52). In trial II (2003-2007), the mortality rate was 4.2 per 1000 person-years among children vaccinated in presence of maternal measles antibody and 14.5 per 1000 person-years without measles antibody (MRR, 0.29; 95% CI, .09-.91). Possible confounding factors did not explain the difference. In a combined analysis, children who had measles antibody detected when they received their first dose of MV at 4-6 months of age had lower mortality than children with no maternal antibody, the MRR being 0.22 (95% CI, .07-.64) between 4-6 months and 5 years. CONCLUSIONS: Child mortality in low-income countries may be reduced by vaccinating against measles in the presence of maternal antibody, using a 2-dose schedule with the first dose at 4-6 months (earlier than currently recommended) and a booster dose at 9-12 months of age. CLINICAL TRIALS REGISTRATION: NCT00168558.

Two different doses of supplemental vitamin A did not affect mortality of normal-birth-weight neonates in Guinea-Bissau in a randomized controlled trial.

Whether neonatal vitamin A supplementation (NVAS) should be policy in areas with vitamin A deficiency is debated. We observed that a smaller dose of vitamin A may decrease mortality more than a larger dose and conducted a randomized, double-blind, placebo-controlled trial in Guinea-Bissau with the primary aim of comparing the effect of 50,000 with 25,000 IU neonatal vitamin A on infant mortality. The secondary aim was to study the effect of NVAS vs. placebo, including a combined analysis of NVAS trials. Between 2004 and 2007, normal-birth-weight neonates were randomly assigned in a 1:1:1 ratio to be administered 2 different doses of vitamin A (50,000 or 25,000 IU) or placebo. Infant mortality rates (MRs) were compared in Cox models providing MR ratios (MRRs). Among 6048 children enrolled, there were 160 deaths in 4125 person-years (MR = 39/1000). There was no difference in mortality between the 2 dosage groups: the MRR for 25,000 vs. 50,000 IU was 0.96 (95% CI: 0.67, 1.38). Neither dose of NVAS was associated with lower mortality than placebo (MRR = 1.28; 95% CI: 0.91, 1.81). In a combined analysis of the present trial and 2 previous NVAS trials in Guinea-Bissau, the effect of receiving NVAS (any dose) vs. placebo was 1.13 (95% CI: 0.94, 1.36) and differed significantly (P = 0.01) between boys (0.80; 95% CI: 0.58, 1.09) and girls (1.35; 95% CI: 1.04, 1.75). We could not confirm that a smaller dose of neonatal vitamin A reduces mortality more than a larger dose. We confirmed 2 other trials in Guinea-Bissau that showed no beneficial effect of NVAS. This trial was registered at clinicaltrials.gov as NCT00168610.

Live vaccine against measles, mumps, and rubella and the risk of hospital admissions for nontargeted infections.

IMPORTANCE: In low-income countries, live measles vaccine reduces mortality from causes other than measles infection. Such nonspecific effects of vaccines might also be important for the health of children in high-income settings. OBJECTIVE: To examine whether the live vaccine against measles, mumps, and rubella (MMR) is associated with lower rates of hospital admissions for infections among children in Denmark. DESIGN, SETTING, AND PARTICIPANTS: Population-based cohort study of Danish children born 1997-2006 and followed up from ages 11 months to 2 years (last follow-up, August 31, 2008). Nationwide Danish registers provided data on vaccinations and hospital admissions. The recommended vaccination schedule was inactivated vaccine against diphtheria, tetanus, pertussis, polio, and Haemophilus influenzae type b (DTaP-IPV-Hib) administered at ages 3, 5, and 12 months and MMR at age 15 months. MAIN OUTCOMES AND MEASURES: Incidence rate ratios (IRRs) of hospital admissions for any infection, comparing receipt of MMR vs DTaP-IPV-Hib as the most recent vaccine. Risks, risk difference, and number needed to vaccinate were calculated for receiving MMR on time. RESULTS: The study included 495,987 children contributing with 56,889 hospital admissions for any type of infection during 509,427 person-years (rate, 11.2 per 100 person-years). For the 456,043 children who followed the recommended schedule and received MMR after the third dose of DTaP-IPV-Hib, MMR (rate, 8.9 per 100 person-years) vs the third dose of DTaP-IPV-Hib (rate, 12.4 per 100 person-years) as the most recent vaccine was associated with an adjusted IRR of 0.86 (95% CI, 0.84-0.88) for any admission for infection. There were 19,219 children immunized out of sequence. The adjusted IRR was 0.87 (95% CI, 0.80-0.95) for those receiving MMR (rate, 9.9 per 100 person-years) after the second dose of DTaP-IPV-Hib (rate, 15.1 per 100 person-years). However, in the 1981 children who subsequently received the third dose of DTaP-IPV-Hib (rate, 12.8 per 100 person-years) after MMR, the IRR for hospital admissions for infection was significantly greater (adjusted IRR, 1.62 [95% CI, 1.28-2.05]). The risk of admission for an infection between ages 16 months and 24 months was 4.6% (95% CI, 4.5%-4.7%) for receiving MMR on time and 5.1% (95% CI, 5.0%-5.2%) for not receiving MMR on time. The risk difference was 0.5 percentage point (95% CI, 0.4-0.6), and the number needed to vaccinate with MMR before age 16 months to prevent 1 admission for any infection was 201 (95% CI, 159-272). CONCLUSIONS AND RELEVANCE: In a cohort of Danish children, receipt of live MMR vs inactivated DTaP-IPV-Hib as the most recent vaccine was associated with a lower rate of hospital admissions for any infections. These findings require replication in other high-income populations.